Sammy Keyes and the Runaway Elf by Wendelin VanDraanen

Thirteen year old Sammy is on yet another mystery solving adventure. Rich Mrs. Landvogt's dog has been kidnapped and Mrs. Landvogt has threatened Sammy to find her precious little dog or ELSE she will make some real trouble for Sammy. During Sammy's hunt for Merek the little pooch, Sammy comes upon a little girl, Elyssa who keeps running away from home. Elyssa has had some sadness in her life and needs a friend.

If you haven't yet read a Sammy Keyes mystery, this would be a good one for starters! You will want to read another when you finish!