The Dalglish autobiography sold 200,000 copies but Keegan's publisher Little Brown expect him to do even better.
Alan Sampson, who is overseeing the deal for Little Brown, said there was "a feeding frenzy" among publishers in the rush to sign Keegan.
Sporting autobiographies of major stars sell like hot cakes. The cricketer Ian Botham was one of the biggest sellers of 1994 with his autobiography and My Championship Year by the motor racing star Damon Hill sold 140,000 copies at pounds 25 each.
Keegan, the former England captain with 63 international caps, two European Footballer of the Year trophies and his years in the hot seat at Newcastle United, has a lot to write about.
The autobiography is expected to be published in October, followed by a book of humorous reminiscences and another on coaching.
Sampson told the publishing industry paper The Bookseller that he had been tracking Keegan for three years: "But a big name isn't enough - mediocre works quickly fall by the wayside."Reuse content