Stuart Little by E. B. White
A family has a child that just happens to be a mouse. He has some challenges with his small size. He later makes friends with some of the small animals, such as a bird and has to watch out for the cat. After a bird goes missing, he goes on a quest to find her, using a mouse sized car. The book treats the fact that he is a mouse as perfectly normal. There are plenty of fun bits related to life as a small being small.
