A world without the Beatles is a grim place, especially if your name is John Lennon. In the alternate universe of Larry Kirwan's novel LIVERPOOL FANTASY (Thunder's Mouth, $14.95), the band disintegrates in 1962, and, a quarter century later, John is Liverpool's town drunk, bitter both about his hangovers -- ''dawn snuck through a tear in the curtain and threw up all over the ducks on the wall'' -- and his former writing partner's success under the nom de schmaltz Paul Montana. Meanwhile, Father George Harrison, S.J., has some grave doubts about his career, and only Ringo has much dignity -- if the word is not too grandiose to apply to the drummer for Gerry and the Pacemakers. Oh, and the U.K. is overrun by the right-wing National Front. Perhaps you have an inkling that the band will reunite for a misty-eyed gig in an endearingly dank pub. Perhaps you sense that some speculative musings precede the climax (John: ''I know this is way beyond you, but we could have changed the bloody world''). You're right, and if this novel were merely about the Four, then it would be far from fab. But Kirwan, a playwright and the frontman for the band Black 47, spins it as a realist tale of friendships neglected and dreams deferred, featuring shockingly credible versions of personas you know and love. Only Mean Mr. Mustard could sneer.