What kind of person writes a book about Arctic wildlife, 18th-century surgery or the byways of Elizabethan poetry? Most of the readers, one might assume, will be within universities, so who will the authors be if not academics? And in general, no doubt, that assumption will be correct. Yet, just as many 19th-century country clerics produced important work on natural history, one can still find examples of "independent scholars" - people unattached to universities who venture more or less knowingly into academic territory.

Take the case of Richard Sale. He studied physics, stayed on to do a PhD and then worked in the nuclear industry until 1996, when he began to focus his efforts on writing and photography. He has now written more than 60 books, the bulk of them travel and walking guides covering fairly familiar territory such as Dorset and the Italian Lake District.

Yet Sale himself is also a more adventurous traveller, particularly in the Arctic, which he has now visited more than 30 times. He has been dropped off on the vast and virtually uninhabited Ellesmere Island in the Canadian Arctic Archipelago and left there alone for a fortnight, which once required him to bandage up a fractured thumb with Sellotape. Another time, he and a couple of colleagues had to hold down and perform surgery on the leg of a husky that had been badly bitten by another, at night and without anaesthetic, in a temperature 20 or 25 degrees below zero.

Since 2005, Sale has also drawn on such experiences in a series of books: The Arctic: The Complete Story, A Complete Guide to Arctic Wildlife and The Scramble for the Arctic: Ownership, Exploitation and Conflict in the Far North, together with more specialist monographs on particular species, The Gyrfalcon, and The Snowy Owl (due out next year). The last three of these titles were written in collaboration with Eugene Potapov, assistant professor of biology at Bryn Athyn College in Pennsylvania. Although Sale has never had an academic post, those works are in effect academic books, widely reviewed in specialist journals.

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Sale's wildlife guide is the most comprehensive available and draws on observation of virtually every species of Arctic bird and mammal - "from the Siberian tit to the snowshoe hare", according to one review - as well as an extensive survey of the literature. The gyrfalcon is not the easiest species to study, given that it often nests a hundred feet up a cliff, but Potapov and Sale's monograph was published by Yale University Press and given an award by the American Wildlife Society, on the basis of submissions from the largely academic membership. The Scramble for the Arctic was a book of the month in the Royal Geographical Society's Geographical magazine. Another of Sale's co-authored books, on high-altitude physiology, is used on the first English-language diploma in mountain medicine at the University of Leicester.

Few could dispute Sale's claim to the title of independent scholar, if we take that to mean someone who works outside the university sector but produces books or papers accepted as "genuine contributions to knowledge" by the relevant academic community, as attested by citations, reviews, invitations to conferences and appearances on reading lists. For his latest project, which arises out of his observation of Arctic falcons, he has forged a loose unpaid link with the University of Oxford as a senior research associate.

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Independent scholars come in many shapes and sizes. Some work between disciplines, or in disciplines that are not yet fully established, and so have no natural niche within traditional academic structures.

One such is Jonathan Burt. He first became interested in animals in history in the early 1990s, when there were few others exploring this "very quiet" field on the boundaries between biology, social and cultural history. He founded the interdisciplinary Animal Studies Group and became the editor of the Animal Series for Reaktion Books, for whom he wrote the volume Rat as well as Animals in Film. He has also produced about 30 academic articles on topics ranging from primatology to pets.

Animal studies has now become an established discipline, at least in the US, which often draws on Burt's work. Although "at conferences or lectures that I give I am just like any academic as far as the audience is concerned", he doubts whether he would "have been able to work on so many things within a university".

Another example is social theorist and urban geographer Andy Merrifield, who in essence writes for an academic readership but still hopes to put a bit more punch into his prose than journal editors might allow.

He is the author of books such as Dialectical Urbanism: Social Struggles in the Capitalist City, Magical Marxism: Subversive Politics and the Imagination and, most recently, a study of the British novelist and art critic John Berger. Although The Wisdom of Donkeys: Finding Tranquillity in a Chaotic World proved commercially successful, Merrifield has always been more interested in "writing for academics and students in the US and the UK". After 2003, when he gave up an academic post in the US, he lived frugally in France for many years as an independent scholar. He has only recently returned to academe via a 10-month post as a Leverhulme visiting professor of geography at the University of Manchester.

Often recognised when he goes to conferences, Merrifield is "pleased when universities invite me to give talks, as it proves I can cut it in academia". Yet he also sees advantages to an independent status that "trades in salary and institutional respectability for freedom of voice. The content of my books is academic, though the style is more playful and poetic. They tend to relate to what's going on in the world (such as the Occupy movement) rather than the self-referential world of my peers. I am sometimes told off for not engaging with certain texts, but that seems nit-picking when they neither bolster nor undermine my argument."

Nonetheless, Merrifield also acknowledges the difficulties of trying to produce books that combine "heaviness of ideas and lightness of prose", namely that they can end up "too complex for general readers and too breezy for academics".

This brings us to the vexed question of "crossover" books. Most "independents" do not write purely for academic readers, particularly when authorship is their main or sole source of income, so they tend to avoid the more elaborate systems of citation and the colourless, dispassionate style often found in academic texts.

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Yet many are adamant that they are just as scrupulous as "professionals" with the evidence and careful to make clear when they are speculating. Some go further and claim that those with a perch outside the academy can produce better work. A background in journalism may enable them to bring complex issues to life in a way many academics find difficult. Experience of public life may give them an extra level of insight into the political processes investigated by historians or students of international relations.

In the face of such arguments, academics are often concerned to police the boundaries between independent and "proper" research. They can, for example, point to amateur historians who survey mountains of evidence and do vast amounts of work in the archives in the service of a crazed conspiracy theory. Yet there is also a danger that a concern with "rigour" may be a mask for snobbery or for academics protecting their own turf.

Some of these issues were raised in a recent series of acrimonious articles. Last year, Helen King, professor of classical studies at The Open University, wrote in Social History of Medicine about "the genesis of the 'historical fact' that the eighteenth-century men-midwives, William Smellie and William Hunter, colluded in the murder-to-order of women in the final stages of pregnancy, to supply the bodies on which the illustrations for their atlases of obstetrics were based". This claim had been made, she continued, by Don Shelton, "a New Zealand journalist and collector of miniature portraits", in an article "based on a chapter in [his] self-published e-book The Real Mr Frankenstein, a biography of the 19th-century [surgeon] Sir Anthony Carlisle". It had gone on to attract wide media attention because it "appeal[ed] to the public appetite for stories about fallible founding fathers, doctors who transform into murderers, and the flaws of scientific medicine".

Shelton, clearly irked by the way he had been described, responded by arguing that King was "taking a stance against 'amateur' or 'new' historians", when their work deserved "to be judged on its merits". Many of the old barriers had come down, he added, now that "digital resources allow any historian to conduct original research by remotely accessing source material which previously required physical access".

In rebuttal, King denied taking any such stance: "My concern is instead 'bad' history, on the model of Ben Goldacre's 'Bad Science'. 'Bad' history can be produced within the academy: 'good' history can be done by those who do not work within university History departments." Although she was strongly critical of Shelton's methodology, she had nothing but praise for other "independents" such as Wendy Moore, whose first book, The Knife Man: Blood, Body-snatching and the Birth of Modern Surgery, had "been justly acclaimed not only as a biography but also as a compelling portrait of the period".

Moore is a medical journalist who does not consider herself a historian and does not even have a degree, although she obtained a one-year Diploma in the History of Medicine from the Society of Apothecaries. The Knife Man is a biography of the distinguished surgeon John Hunter (1728-93). A fictionalised version is now being filmed as a pilot for a television series by David Cronenberg.

In researching the book, Moore explains that she "read a lot of academic work as well as primary sources, sifting through huge amounts of archives at the Wellcome Library and the Royal College of Surgeons. I often went to experts if I didn't understand anything. Some academics are very supportive of outsiders who are interested in their field, but others see them as a threat."

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Although she "makes no apologies for the title" and was keen "to bring medical history to doctors and to bring John Hunter to a popular audience", Moore was equally concerned "to be taken seriously by academics. I was absolutely scrupulous about not going beyond the evidence, even when referring to the weather. I wanted to have a strong narrative while always referring to the evidence." This is, of course, a difficult tightrope to walk, but journal reviews, conference invitations and the acclaim of King and others suggest that it is not an impossible task.

Other independent scholars are less constrained by the need to make a living and rely on pensions, private incomes, supportive partners, freelance work or a parallel day job rather than book sales to keep body and soul together.

A celebrated example is Lord (Jonathan) Sumption. He briefly pursued a career as an academic historian but, after being called to the Bar in 1975, went on to become one of the country's leading barristers. He has represented everybody from the Queen to Roman Abramovitch, and he was appointed a Justice of the Supreme Court at the beginning of this year. Yet he is also at work on a meticulously researched five-volume narrative history of the Hundred Years War, which reached its third volume in 2009. His income as a lawyer, he once said, meant that he "can work more productively than any academic because of my large personal library - about 7,000 volumes on the Hundred Years War alone".

Sir Rodric Braithwaite, who is 80 this month, was British Ambassador in Moscow from 1988 until 1992 and a foreign policy adviser to prime minister John Major for another year after that. He then served as an adviser to Deutsche Bank and chair of the Royal College of Music and sat on the board of the English National Opera before retiring properly when he reached the age of 70. On the very same day, his first book was published: Across the Moscow River: The World Turned Upside Down. This was a sort of political memoir, drawing largely on his diaries, although it has "got into bibliographies and on to the reading lists of obscure American university courses".

Much of Braithwaite's seventies was devoted to the research and writing of two further books: Moscow 1941: A City and Its People at War, published in 2006, and Afgantsy: The Russians in Afghanistan, 1979-89, which came out last year. Moscow 1941, he says, "went into 18 languages, including Indonesian, Japanese, Chinese and Hebrew. I think that is because there is a worldwide audience of anorak-clad nerds for books about the Second World War. But it has also got itself into student reading lists and scholarly footnotes."

Afgantsy was mainly intended by Braithwaite "to draw a more objective picture of that war, and the people involved in it (with whom I mostly sympathised), than used to figure in Western writing". It has also inevitably attracted interest because of the dispiriting parallels with the current Anglo-American campaign. Braithwaite reports that it "is being read by scholars, soldiers, diplomats and journalists - all categories have been in touch with me...It too is already figuring in footnotes."

Although he would not describe himself as a scholar, he is eager to ensure that "every sentence is justified and to make clear when I am speculating without destroying the prose", sometimes by sticking the caveats away in footnotes where they may go unread. Finances permitting, he says, he sees "every advantage in independence, and no disadvantages".

His diplomatic career included seven years in Russia (and three in Poland), giving him an insider's understanding of the region and its politics rare in British-born academic experts. His first-hand experience of diplomatic writing was also helpful.

"I have written official documents," explains Braithwaite, "and I know they are misleading - something historians don't always realise."

Historians "often over-interpret something they find in a single document. They can also fail to understand the nature of decision-making in politics and war. It's never very rational. There are many unknown factors, and you're always walking into the darkness - rather like when you take the decision to get married. I try to bring empathy to my study of individuals, who are always driven more by emotion than calculation."

However diplomatically, Braithwaite touches on some of the important issues independent research raises for what one might call mainstream academic writing. The simplest and most benign view is that knowledge is knowledge, and that outsiders are simply producing a useful addition to what is being created within universities. What makes the whole phenomenon far more significant is that they can also be seen as a distraction, an irritation and a challenge.

The outsider's eye: 'I put in anything that interested me, even if was very elementary, because it was fresh to me'

Author Nicola Shulman sees Graven with Diamonds: The Many Lives of Thomas Wyatt: Courtier, Poet, Assassin, Spy, published last year, as "a non-academic book on an academic subject", and so was "very conscious that it might fall completely into oblivion".

Who on earth would want to read a full-length study of a minor Elizabethan poet now "very out of fashion" even among the small number of people interested in such things?

She seems to believe that career academics are rigorously professional and describes her own approach, in contrast, as cheerfully amateurish.

"I can't read early 16th-century orthography," she says, "so I had to use only written sources. I had a wonderful time in the company of the great Tudor historians and probably read quite a lot of books that are now considered old-fashioned. If you're an outsider, you don't know what scholars would be expected to read.

"I didn't always know where to find things, used indexes to get me from book to book, and was particularly lamentable in looking for things I knew I had but couldn't find again. I put in the book anything that interested me, even if was very elementary, because it was fresh and not jaded to me coming to it as a general reader."

Shulman feels that there are advantages as well as disadvantages in "not being constrained by what is acceptable and fashionable in a university" and "being effectively beneath the radar of competitive academic criticism". Although she made every effort to get her facts right and befriended an academic expert, she was chiefly concerned to produce a readably idiosyncratic book that could function as "an introduction to the whole literature of the time" but also incorporate "an element of detective story, taking place against backdrop of what (the novelist) Hilary Mantel called 'our national soap opera'. I wanted to present a familiar object from an unusual angle."

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In the event, Graven with Diamonds got rave reviews - one of them noting its "forensic acuity" and "quite spectacular intelligence" in teasing out "the threads of romantic intrigue, factional conspiracy and theological dispute" that left their mark on Wyatt's poetry - and it enjoyed excellent sales.