In 1970 Laurence James became an editor at the paperback publishing house, New English Library. It was the beginning of a career that saw him become what he described as the country's "least known bestseller", writing, under pen-names, 12 to 14 novels every year, 160 in total, which sold more than 12m copies.

The NEL he joined handled a list ranging from Harold Robbins, Irving Stone, Frank Yerby and Dorothy L Sayers to a line of original novels that exploited the tabloid headlines of the day. In 1970, the company published the bestseller Skinhead, by Richard Allan (James Moffatt), but the real boom began a year later, when William Terry's novelisation of the spaghetti western, A Town Called Bastard, inspired a series starring Edge, the Loner, written by Terry Harknett (as George G Gilman). That year too, Peter Cave's hell's angel novel, Chopper, appeared.

Only recently discovered by academics, but longtime cult favourites for collectors, these million-sellers inspired a rash of imitations. Since NEL was, as James admitted, "the bottom end of the market, we were either creative or we went under".

James left NEL in 1973 to freelance for the market he had helped create. He wrote hell's angel novels as Mick Norman; Viking and Roman historical novels as Arthur Frazier, Andrew Quiller and Neil Langholm; science fiction under his own name; humorous confessions as Jonathan May and Christopher Nolan; and romances as Mary Fraser. He produced dozens of westerns, beginning in 1974 with the Apache series (as William M James), a violent revenge story which ran over two dozen novels, as did the Herne The Hunter series for Corgi (written as John J McLaglen). Many of James's pseudonyms were shared between a small, alternating group of writers; for example, William M James was split between James, Harknett and John Harvey.

The paperback boom lasted only five years, after which James turned to horror novels, writing as James Darke and Richard Haigh, juvenile books (some in collaboration with his son, Matthew) about American football and baseball, and a children's fantasy series. In 1986, as James Axler, came his most successful series, the Darklands books. Set a generation after nuclear war has devastated America, they were grim adventures, imaginatively written. In any of the three dozen titles that James wrote, Vikings or General Custer were as likely to show up as mutants or survivalists.

After grammar school, Harris trained as a teacher at Goldsmith's College, south London, found work as a scaffolder, and later sold books at Foyles and Harrods, before entering publishing with Leslie Frewin in 1968. In the mid-1990s, he was diagnosed as having cancer, which he endured with honesty and humour. He is survived by his wife, the writer Elizabeth James, two sons and one daughter.

• Laurence William James, author and editor, born September 21 1943; died February 10 2000