Philip Horne's first exposure to Henry James came when he read The Golden Bowl as an undergraduate at Cambridge in 1976. "I read about five pages an hour and it took three weeks," Horne recalls. "It was a total immersion." A quarter of a century on, Horne is still immersed. His book Henry James and Revision was published in 1990 and he has edited new editions of three James novels.

This month sees the paperback publication of his Henry James: A Life in Letters . As the book evolved over 10 years, Horne realised that he was "fashioning a Victorian life and letters. I felt that the letters were very close to making a narrative you could follow. I just needed to fill in the gaps."

Through the use of links and introductory paragraphs, Horne builds a convincing case for the letters being, in James's phrase about his brother William's letters, "his real and best biography". They are packed with contacts with the leading literary figures of the age - Flaubert, Zola, Turgenev, Thackeray and Ezra Pound all crop up - and contain some of James's major writing. "There are passages that are quoted as much as anything else he wrote," says Horne.

There is no complete edition of James's letters, and about half of the 296 letters in Horne's book have not been published before. The consensus is that James wrote between 12,000 and 15,000 letters in his lifetime, but this is guesswork: it could be as high as 40,000. "I'm not a vocational editor," Horne explains. "I edited them because I felt that someone should and no one else would. I wanted them to be there so I could read them, so this is really the book I wish somebody else had done for me to refer to."

Unlike many biographers who have worked on a subject for such a long time, Horne has not grown to hate James. He is proud to be the owner of one James's letters to a magazine editor. ("I was a prisoner in London all winter with gout and fetters," James complains in it.) "A lot are still in private hands and collectors often don't want them published because it reduces their value," explains Horne. "I paid £400 specifically so I could give myself permission to include it. I'd like a portrait of James, but they are rather more expensive."

Academic study of James has in recent years been energised by speculation as to James's sexuality. Horne makes no judgment but acknowledges: "I know others are more eager to read between the lines than me. I was interested in James as, primarily, a creative writer. I thought attention to things that we could definitely know was more rewarding than being preoccupied with things we could only guess at, however interesting they are. But it's not an area I would rule out. There is no question that James knew lots of gay writers and gay men and was on close terms with them. But he was on close terms with lots of people."

One area where Horne has offered a new reading is that of James's relations with the business of publishing. "It rather complicates the picture of him as a helpless, unworldly artist in his ivory tower - although that has some truth to it," reports Horne. "He could also be quite hard-nosed and tough, and some of the letters to publishers and editors are fairly firm and even aggressive."

Horne's next James project takes this theme on, and will further explore James rubbing up against various harsh aspects of the real world - "James on American politics, the theatre and on the critical controversies of the 1880s," says Horne. "It was quite a knockabout world of polemic and abuse through which he tried to pick his way."

Although best known through his work on James, Horne has also managed to engage in a wide range of other literary activities. He has been writing literary journalism since he started reviewing new fiction for the London Review of Books in 1983. As Reader in English Literature at University College, London since 1995, he says he has done as much teaching of film - he writes for the film journal Sight and Sound - as he has of Henry James.

"I very much enjoy teaching. When you can get it right, the balance between teaching, writing and journalism can be very good. That balance between solitude and human contact can be the ideal. It keeps your feet on the ground and reminds you what people are interested in."

His next publication will be an introduction to the new Penguin edition of Oliver Twist . "Very good fun. A refreshing change from James - and it's nice to work on a best-seller."

What about the future of letters? Will technology kill off this literary resource? "Well, someone will need to print out emails in order for them to survive," say Horne. "The electronic form is always liable to be cleaned out or the software won't be supported. Paper is still the primary medium, and I certainly print out the interesting emails I get."

One way in which email will help is by solving the problem of indecipherable handwriting. By re-examining the originals, Horne has uncovered several mistakes in previous editions of the letters that alter the sense of what James actually said.

"But while emails will of course make it easier, it will also be less exciting," says Horne, whose enthusiasm for tracking down odd scraps all over the world appears undiminished. "It just won't be the same as a scholar in the archives looking at crumbling old letters written in mauve ink with an actual signature on the end."