Second grader Leigh Botts adores a book about dog training by author Mr. Boyd Henshaw. To fulfill a school assignment, Leigh painstakingly writes to Mr. Henshaw and tells him how much he "licked" his book. And he keeps writing to him.

At first, Leigh resents the flippant replies he gets from his idol, but gradually he answers Mr. Henshaw's questions, which are really only to encourage Leigh to put pencil to paper. Four years later, Leigh is in the sixth grade and is still writing to Mr. Henshaw, and he never imagines that he'd reveal so much of himself and his life to the author:

"Dear Mr. Henshaw, I am sorry I was rude in my last letter... Maybe I was mad about other things, like Dad forgetting to send his child-support payment..."

Gradually not only his spelling and writing improve, but also his feelings about life, as Mr. Henshaw inspires Leigh to keep a journal.

This Newbery Award-winning book by veteran author Beverly Cleary traces the inner life of a sometimes lonely and often troubled boy during that all-important but painful transition into adolescence, and she does so with grace and humor.
source: https://shop.scholastic.com/teachers-ecommerce/nys/books/dear-mr-henshaw-9780590687270.html