The literary career of A.(lan) A.(lexander) Milne (1882-1956) ranged over five decades and included everything from humorous essays and light verse to political pamphlets, novels and a long philosophical poem. Before the advent of "Winnie-the-Pooh," Milne was best known as a successful playwright, whose sparkling adult comedies delighted London audiences looking for diversion in the wake of World War I. "When there is nothing whatever to say," remarked one critic, double-edgedly, "no one knows better than Mr. Milne how to say it."

But it was the four children's books about a boy and his bear written in the 1920s that would win him the most lasting fame. Two were collections of light verse: "When We Were Very Young" (1924) and "Now We Are Six" (1927); the other two - "Winnie-the-Pooh" (1926) and "The House at Pooh Corner" (1928) - were collections of stories about the eponymous Bear of Very Little Brain, his fellow stuffed animals, and their resourceful child owner, Christopher Robin.Milne had a happy start in life, even if it was far from the complacent, upper-middle-class upbringing some of his critics (including his son) later accused him of having enjoyed. He was the youngest and brightest of the three sons of J.V. Milne, the enlightened headmaster of Henley House. At Cambridge University, Alan got a head start on his literary career, editing the student magazine Granta. Short on contributions, he proceeded to fill the gaps with light, amusing pieces of his own. After graduating with a disappointing "Third," he set out to make his way in the literary world, with the usual difficulties. He eventually found his way to the British satirical magazine Punch.

Milne was aware of the gap between his childhood precocity and his later performance. All the stories about his infant feats, he later declared, would have been more appropriate if told about a 2-year-old Abraham Lincoln.

In 1913, Milne married Dorothy de Selincourt ("Daphne" or "Daff"). Although it lasted until death parted them, it was not a union of soul mates. Each sought other outlets for their affections - Dorothy with the American playwright Elmer Rice, Alan with actress Leonora Corbett. Thwaite, who is nothing if not cautious, furnishes the known facts and leaves readers to draw their own conclusions about the extent of these involvements.

Thwaite, author of two previous, well-received biographies (of the Victorian man-of-letters Edmund Gosse and of children's writer Frances Hodgson Burnett), portrays Milne as a man and a mind worth taking seriously. She provides detailed accounts of his political positions, gleaned from his pamphlets and the numerous letters-to-the-editor he got into the habit of writing. She also believes that his plays, for all their graceful frivolity, have interesting things to say about modern marriage and other relationships, but she does not provide full enough accounts of their plots to buttress her argument. She writes insightfully about his personality and intellect. She is less successful in conveying a sense of what his everyday life was like. One also misses the convenience of a chronology.

Herself an author of children's books, Thwaite is sensitive to the special talent necessary to produce the seemingly effortless lightness that characterized so many of Milne's writings, for adults as well as children. Without making extravagant claims for him, she has written an engaging and thoughtful biography that gives Milne his due.