by Robert Nye. Little, Brown, $8.95. It is Mr. Nye’s amusing conceit that Sir John Falstaff, having apparently lived to the age of 150 or so, dictates his memoirs to a gaggle of browbeaten secretaries. The old rascal’s style shows understandable traces of Chaucer and a strong influence from the unborn Rabelais. In short, the stuff is witty, outrageously slapstick, amiably bawdy, and almost totally unbelievable. But it has a serious undercurrent, summarized by the only amanuensis with the courage to disobey the master’s orders: “He writes a kind of requiem for a life he never lived. . . .”