Cam Jansen, The Mystery of the Stolen Diamond, by David Adler, Puffin Books, 1980.
My second grade son read this book for his Literature class, and learn to make inferences and deductions using this book.
This book was my son's first introduction to a mystery story. It wasn't difficult for him to figure out, and he enjoyed it. But I can't be the only mom who nearly had a heart attack when the baby was brought into the 'den of thieves'.