The Case of the Disappearing Dinosaur (Jigsaw Jones Mystery, No. 17) by James Preller

Jigsaw Jones is a puzzle fanatic second-grade detective who deals with all kinds of cases. He knows that mysteries are like jigsaw puzzles-you've got to look at all the pieces to solve the case. Missing hamsters, lost coins, haunted houses...Jigsaw Jones has seen it all before. With his top secret detective journal, eye for detail, and ace partner Mila, Jigsaw is always ready to take on a new case. Now you see it, now you don't! Danika is doing a magic act at Bigs's birthday party. For her biggest trick, she makes Bigs's favorite toy disappear. Poof--it's gone! But where did it go? Even Danika doesn't know. Time for Jigsaw and Mila to use their own tricks and fix this mixed-up magic.