Stink: The Incredible Shrinking Boy by Megan McDonald

Stink is the shortest person in his family.  He's the shortest kid in the second grade.  He's probably the shortest person in the whole world.  Then one day when he's measured, he's a quarter inch shorter.  Could he actually be shrinking?

This book is the first in a series about Judy Moody's younger brother Stink.  It is a great series of books for boys (and girls) ready for chapter books.  

As an added bonus, it is also available in an audiobook version narrated by Nancy Cartwright, voice of TV's Bart Simpson.