You know how it is: you wait ages for a superb study of the pervasive paranoia that formed the dark heart of the Tudor state and then three come along at once.

First, in the autumn of 2011, we had Thomas Penn's Winter King: The Dawn of Tudor England, a stunning exploration of the fear-driven regime of Henry VII, the usurping founder of the dynasty. Next came John Cooper's The Queen's Agent: Francis Walsingham at the Court of Elizabeth I, an absorbing portrait of the man who was not simply secretary to Henry VII's granddaughter Elizabeth, but her master of spies. And now we have Stephen Alford's The Watchers, an enthralling account of the murky shadow-world of Elizabethan espionage over which Walsingham presided.

Luckily for us, these riveting tales - all told by authors at the top of their game - are complementary rather than competing narratives. Penn took us into the rarefied atmosphere inhabited by an insecure king at the end of the 15th century, and Cooper to the top table of government under a differently but equally insecure queen in the second half of the 16th. The fascination of Alford's book, by contrast, lies in its focus on the worker bees in the intelligence hive. He has delved deep into encrypted archives to discover the lengths to which Elizabethan Englishmen were prepared to go to destroy their queen, or to defend her - and one of the surprises of a story full of dizzying twists is quite how many of them ended up attempting to do both.

The key to it all was faith. Elizabeth was the living embodiment of the English Reformation, the baby born to Anne Boleyn after Henry VIII had broken with Rome in order to put aside his first wife, the Spanish princess Catherine of Aragon. When Elizabeth became England's queen, therefore - her accession itself a whiplash-inducing turn of fortune's wheel after the unforeseeably early deaths of her Protestant brother Edward VI and her Catholic sister Mary - she was denounced by Catholic Europe as a bastard, a heretic, a schismatic and a tyrant.

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All of which left English Catholics in something of a fix. Should they keep their heads down and try to find some fraught accommodation between their loyalty to a queen who claimed to be supreme governor of a Church they didn't recognise and their devotion to a Church that didn't recognise her right to be queen? Or should they take up arms to answer the Pope's call to depose her? Meanwhile, the quandary for Elizabeth's government was equally pressing: how best to secure the future of Protestant England when Catholic enemies were everywhere at hand, within and without England's borders.

The Watchers begins with a reminder that then, as now, being paranoid doesn't mean they're not out to get you. (Or, as Walsingham elegantly put it, "There is less danger in fearing too much than too little.") In a bravura piece of counterfactual storytelling, Alford describes the moment in an imagined 1586 when one of the many plots to assassinate Elizabeth finally succeeded, leaving England to be overrun by the invading forces of Catholic Spain and the Elizabethan age to go down in history as a fleeting and futile attempt to resist Habsburg domination.

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This was the doomsday scenario that haunted Elizabeth's ministers - and it took on a nightmarish intensity from the early 1570s. It was in 1570 that Pope Pius V excommunicated the English queen, thereby freeing her Catholic subjects from their duty of obedience to her. Two years later, the horror of the St Bartholomew's Day massacre in Paris emphasised for English Protestants the terrifying proximity of the Catholic Antichrist. The unmarried queen, meanwhile, was fast approaching 40; not only did her childlessness mean that contemplating the political future felt like staring into the abyss, but her most plausible heir, her Catholic cousin Mary, Queen of Scots, was under lock and key in England, a talismanic, magnetic presence for Catholic conspirators.

And there were plenty of them. Catholics in exile on the Continent could be assumed to be hostile to Elizabeth's regime, and Catholics in England a threatening fifth column, while from seminaries in Douai, Rheims and Rome - all under the auspices of the inspirational Dr (later Cardinal) William Allen - came the "storm-troops" of the mission, priests primed and ready to save English souls from the tyranny of the English Jezebel. But before they had set a heavily disguised foot on England's shore they were watched, from afar and from close at hand, by men prepared to take equal risks to thwart them.

Among these watchers were Anthony Munday, an aspiring writer and self-publicist who became a spy when the opportunity fell into his lap on a visit to Rome; Charles Sledd, a silently forensic observer, both sharp and sly, later the most ferocious priest-finder in London; and William Parry, whose attempt at a career in espionage was fuelled by deluded vanity. The most impressive of this motley crew was Thomas Phelippes, a thoughtful, reserved and brilliant man whose supreme gifts as a cryptanalyst were invaluable to his master Walsingham.

These "intelligencers", for all their differences, were volunteers. Others were trapped into service: Catholics plotting against Elizabeth who, when discovered, found that they could countenance the prospect of changing allegiance from Church to queen more easily than the agony of the rack or the torture of a traitor's death. Such a one was Gilbert Gifford, a young man whose role as a key courier in the Babington Plot - a conspiracy in 1586 to kill Elizabeth and replace her with the imprisoned Mary, Queen of Scots - was transformed into that of a key double agent, giving Phelippes and Walsingham access to Mary's correspondence and, in the end, helping to send her to the block.

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The burden of fear under which men such as Gifford laboured is breathtaking. They faced the constant need, with no more than partial and provisional information, to calibrate and recalibrate their own status, and that of everyone around them, as pawns or players in a game that put at terrifying risk both their life and their immortal soul.

And they did so within a state-run surveillance operation that we discover, near the end of Alford's story, was not state-run at all. When Walsingham, the ¨¦minence noire of this covert world, died in 1590 - just three years after the Scottish queen's execution and two after the attempted invasion of the Spanish Armada - his intelligence network died with him. Spies, it turns out, were recruited and run by means of private patronage. Some were not paid at all; the cost of others threatened to bankrupt their spymasters. It wasn't until the emergence of the formidable (and formidably wealthy) Robert Cecil as her leading minister in the last years of her reign that Elizabeth was again protected by a secret service to rival Walsingham's.

Walsingham and Cecil have walk-on parts in this narrative from the coal-face of "spiery"; Elizabeth herself is an overshadowing presence who remains always just out of reach for would-be assassins and loyal intelligencers alike. And yet her compulsive fascination exerts its pull throughout these pages. One moment Anthony Babington is pledging body and soul to the task of killing her; the next, thinking it might after all be safer to inform on his co-conspirators, he is picking out his best clothes in excitement at the prospect of meeting her. The heart of the Tudor state, as Alford compellingly shows, is entirely human in its darkness.

THE AUTHOR

A specialist in the politics of England in the second half of the 16th century, Stephen Alford has been appointed to a chair in early modern British history at the University of Leeds. He takes up his post in the 2012-13 academic year, and comes to Leeds from the University of Cambridge, where he spent 14 years as an academic, most recently as a senior lecturer.

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Alford completed his doctoral studies in 1996 at the University of St Andrews under the supervision of eminent Tudor historian John Guy. The work undertaken in his PhD thesis would inform his first monograph, The Early Elizabethan Polity: William Cecil and the British Succession Crisis, 1558-1569, which was published in 1998.

In 1997, he took up a British Academy postdoctoral research fellowship in the Faculty of History at Cambridge, and became a research fellow at Fitzwilliam College. Two years later, Alford was awarded an Ehrman senior research fellowship at King's College. He was named assistant lecturer in the Faculty of History in 2000, followed by an appointment to a lectureship in 2003.

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He is also author of the critically acclaimed monograph Kingship and Politics in the Reign of Edward VI (2002), which Guy called "bold, even radical, in its determination not to be distracted by conventional narratives of politics". Alford is a fellow of the Royal Historical Society.