Something Queer at the Birthday Party by Elizabeth Levy, illustrated by Mordicai Gerstein, 1989.

Jill and some other friends are throwing a surprise birthday party for Gwen. They send out invitations secretly and create a mystery game for Gwen with a series of clues to lead her to the party.  As part of the game, Jill pretends that someone broke into her house and stole a painting of her and her dog, Fletcher.  The clues to the thief lead Gwen into the room where all of their friends are hiding, waiting to surprise her.

Gwen loves their little mystery and the surprise, and the party seems a success. But when it’s time to bring out the cake and open presents, Jill discovers that they are missing from the closet where they were hidden. At first, Gwen thinks that it’s just another mystery game, but Jill tells her that the presents really are missing. Who took the presents, and where are they now?

Gwen takes her party and all the guests on a hunt across town for her missing presents, but the big clues turn out to be right back where they started, and Fletcher leads them right to what they’re looking for.

The solution to this mystery touches on a problem that children with a birthday near Christmas understand: When your birthday is near a major holiday, people don’t pay as much attention to it or give as many presents.  My grandmother said it was the same with her when she was young, and she was also the eldest child in a large family, so her birthday was never quite what it should have been.  In the story, one of the girls’ friends had this problem, and it was the motive behind this temporary theft.  The culprit didn’t mean to keep the presents forever, they just wanted to pretend that they were all theirs for a little while.  Gwen forgives the culprit and shares the rest of her birthday with this person.

The book is part of the Something Queer Mysteries.