Learning about life by letter-writing; Dear Mr. Henshaw, by Beverly Cleary, illustrated by Paul O. Zelinsky. New York: Morrow Junior Books. 144 pp. $8. Ages 8-10.

Veteran author Beverly Cleary just possibly may have been inspired by her many young fans in her 27th book for children. Her latest is a compilation of letters from a boy to his favorite author.

In second grade Leigh Botts reads his first thick book, ''Ways to Amuse a Dog ,'' and writes his first letter to the author, Mr. Henshaw. In third grade Leigh writes another letter for a class assignment and gets a letter back. So, his correspondence begins and grows.

When Mr. Henshaw responds to sixth-grader Leigh's questions about writing books in a letter, he includes his own questions for Leigh. His Mom makes him answer them instead of watching TV, and Leigh's enthusiasm for the friendship cools. He finds that writing is hard work. But he's hooked and hopes to become an author himself when he grows up.

Through the letters we learn that Leigh lives with his Mom in a little house next to a gas station, where he can hear the pings when a car drives in. His Dad drives a big shiny ten-wheel rig and sends child-support checks. He and his Mom scrape by with her part-time wages.

When Mr. Henshaw tells Leigh that he doesn't have time to answer so many letters, he suggests that Leigh get a diary to write in each day. But Leigh can't manage with just blank pages. He finds letter-writing easier, so he continues to write, but now to ''Dear Mr. Pretend Henshaw.''

Through the clever one-way letter format (we never see one from Mr. Henshaw) the story is told. The reader gets to know how Leigh feels about things: his Mom , the divorce, his Dad's absence, his dog, getting along in his new school, a mysterious lunch bag thief, and his writing.

Beverly Cleary has succeeded once more. She's taken a true-to-life situation and woven it into an engaging story.