Graduate students in politics, economics, IT and the social sciences all turn to the same man. Martin Ince meets Manuel Castells

Manuel Castells may go one better in his analysis of the information age than Stephen Hawking in his explanation of the mysteries of cosmology. His mighty trilogy on the new era in which we live has had terrific reviews and has sold well. It is even possible that a large percentage of his readers will get to the end of the volumes instead of (as is famously the risk with Hawking) abandoning them part-way through.

The second edition of The Information Age has just appeared, with substantial new material in two volumes: The Rise of the Network Society and End of Millennium . The second, The Power of Identity , remains the same.

Producing the new material, Castells insists, does not mean that he is following the textbook industry business model of new editions at set intervals until the author perishes - and sometimes beyond. Instead Castells, professor of sociology and of city and regional planning at the University of California, Berkeley, points out that the first edition had a fatal, if accidental, flaw that rendered one more version essential. As he says: "The first volume was finished in mid-1996. Much of it is about the growth of the internet. It already existed, but 1995 was the first year of the worldwide web as we know it. Because of this extraordinary transformation, I think a new edition was needed."

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But Castells, who has been in the US since 1979 after 15 years in Paris, is a thinker who absorbs new information in massive amounts but sees it in a theoretical framework of great consistency. "The internet affects the economy, companies, the market and culture," he says, "and the book is 40 per cent new, but it does not have any new theoretical underpinning."

The first edition of The Information Age appeared between 1996 and 1998. "In English there have been 10 reprints of the first edition," says Castells. "It has also appeared in Spanish, French, Chinese, Swedish, and European and South American Portuguese." Negotiations now nearing completion involve German, Korean, Bulgarian, Turkish and Japanese. And the non-English markets where it has appeared have seen as much reprinting as the English.

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Although the trilogy has attracted wide interest, most of the sales are to academic audiences, with business and general readers a minority. Most of the users, Castells believes, are graduate students. "It is not a textbook for a course," he said during an interview this year in Oxford, where he was delivering the Clarendon lectures to the Said business school (to be published by Oxford University Press). But the books are usable by students of sociology, economics, politics, IT and computer science, and almost all parts of the social sciences.

Technological change fascinates Castells, who sees it at work in his own publishing. One of the reasons the books have been reprinted an impressive number of times is that modern technology enables short print runs and something approaching just-in-time supply. But, he adds, the later publishers have caught on to the books' potential. There was a 10,000 initial print run in France.

Castells is modest about entering the canon of social-science figures. Writing the trilogy cost him a decade of hard work including near-constant travel, reading a huge range of source material and a massive effort of synthesis in which details are not lost. He has been surprised by the impact of the books, which stems, he thinks, from the general feeling of millennial unease and from a feeling that the world has changed fundamentally. Many authors had looked at part of the picture, but Castells can be turned to for wisdom on anything from the web to crime or the collapse of the USSR.

And, he adds, the whole work is empirically based - one reason he dislikes being compared to social theorists. "Anything that does have a sound empirical base is bound to attract attention from people who want to understand what is happening."

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An example is the first volume's material on the world of work, where there is now far more empirical data to support the book's ideas about changes currently occurring. Likewise volume three had to be adapted to cope with Asian economic collapse and rebound, and with events in Europe, especially the former Yugoslavia.

The sheer range of material he covers means that there can be no ideal author for the books Castells produces. He says his own background is interdisciplinary (degrees in economics, law and sociology) and that his early work on urban studies was highly interdisciplinary. His city and regional planning department at Berkeley is open to economic and social approaches as well as to classic planning ones. And he has enough intellectual self-confidence not to be alarmed about switching subjects and approaches. "Exile forces you to be multicultural, and I left for France at 20," he says.

With Franco long gone, he now regularly reappears in his home city of Barcelona. He left France at 26 and, as he put it, proceeded to discover South America and Asia. His arrival in Berkeley in 1979 was followed in 1983 by the publication of a major work on social movements, The City and the Grassroots . This is still a core concern. Virtual communities, Islamic fundamentalism, US militias, the Aum Shinrikyo movement in Japan and the Zapatistas of Mexico are among cases studied in volume two of The Information Age: The Power of Identity .

Moving to northern California gave Castells a ringside seat for the mushrooming of Silicon Valley and led directly to his information trilogy. He realised that the revolution there was greater than anyone in Europe had appreciated. His approach was to analyse "technology interacting with society and the economy" and "to do so multiculturally and internationally". As new pieces of the puzzle appeared not to fit, in Hong Kong, Singapore, Europe and elsewhere, his ideas on new technology altered, so that the work was not "built in a sequence of well-ordered chapters". He thinks computers themselves are none too interesting. It is networks that have the effect. They are as important as the arrival of electricity and the electric motor, by making information available as they did power. But the effects are complex; electricity does not make all countries the same.

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In 1993, the need to complete the trilogy was brought to a head when Castells was diagnosed with cancer and given three years to live. "The project might go on forever, but I realised I would not," he admits. He has had no tumour recurrences for nearly four years and the doctors thanked in Castells's acknowledgments are optimistic.

Now, he says, the genetics revolution promises even bigger changes than the web. But the new edition of The Information Age does not go into the issues it raises at immense length. Castells says: "I am often called the 'philosopher' of this and that, but I am not a philosopher. I am an empirical sociologist. So far biotechnology has mainly happened in laboratories and I do not know how it connects to people, industries or public policy because that has not happened yet. So the human genome project ought to be the subject of another book in five years."

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Martin Ince is editing Conversations with Manuel Castells for Polity Press.