The Slumber Party Secret — Carolyn Keene

WHO'S THE PARTY POOPER?
Rebecca Ramirez's birthday is just two days away, and she's having a slumber party. She made the invitations herself, with rainbow ribbon hair clips and everything. But now the invitations are gone. Somebody took them. And the whole party is ruined!

Or is it? Nancy Drew has promised to help find the missing invitations. Nancy has never solved a mystery before, but she already has a clue or two. Maybe if she writes them down in her special blue notebook...maybe if she thinks real hard...maybe, just maybe, she can make sure that Rebecca's birthday is a party after all!