biography, autobiography, memoirs : A. A. MILNE The Man Behind Winnie-the-Pooh<i> By Ann Thwaite (Random House: $29.95; 554 pp.) </i>

In 1894, Alan Alexander Milne, then 12 years old and already a mathematician of considerable promise, received a report at the Westminster school saying he had “done ill, showing little or no ambition.” It’s hard to believe such a scolding could mark a career turning point, but the future creator of Winnie-the-Pooh, as Ann Thwaite shows in this winning biography, was reared in a child-oriented, child-loving family and was thus quite unused to criticism. The result, wrote Milne years later, was that he “turned my face to the lighter side of life, and abandoned work” . . . a mixed blessing for the writer but not for millions of children and parents the world over.

Thwaite argues--agreeing with Christopher Robin Milne, the writer’s only child--that A. A. Milne was “a mass of contradictions,” being shy yet confident, modest yet proud, warm yet judgmental. The Milne that comes through here is rather less complicated, however, first appearing as a writer thrilled to exercise his talent for whimsy and then, increasingly, frustrated by it.

Milne’s light touch served him well when he edited the Cambridge University magazine, Granta, again as a major contributor to Punch, and even more when he began writing for the stage; by 1924, Milne could marvel that he was making $2,000 a year just from amateur rights to now-forgotten plays like “The Truth About Blayds” and “The Dover Road,” and much more from their long runs in London and New York.

But as he grew older, Milne longed to be taken seriously, both as a writer and a determined pacifist. Yet the enormous success of his children’s books--beginning with “When We Were Very Young” (1924) and “Winnie-the-Pooh” (1926)--made that hope futile.

The influence of Pooh on Milne’s life and reputation may be judged from the fact that references not only to Winnie, Eeyore and Piglet, but even to Owl and Rabbit, have been thoroughly indexed. Christopher Robin is indexed as well, of course, though schizophrenically, the character bearing the same name as the child--a touchy point, Milne being much criticized in recent years for having turned his son into a myth.

In fact, the flesh- and-blood Christopher was known to the Milne family as Billy Moon, a detail that may have blinded the father to the problems that would haunt the son, and which the real Christopher Robin has chronicled in his own books. One gets the sense that the son has made peace with Pooh and his creator, however, for Christopher Robin Milne introduced Thwaite to many of his relatives and has given his blessings to the result. The book is a sympathetic, understanding portrait, and Pooh fans will savor it.