Kathy A.,
other recommendations for an 11-year-old:
Dovey Coe, by Frances Dowell. It has one of the best opening lines of the MS books I read. And I can't find the book, but amazon has it as: >My name is Dovey Coe, and I reckon it don't matter if you like me or not. I'm here to lay the record straight, to let you know them folks saying I done a terrible thing are liars.... I hated Parnell Caraway as much as the next person, but I didn't kill him. A very good mystery.
Time Stops for No Mouse and The Sands of Time, Michael Hoeye. Both of these mysteries are gripping. Hermux is a bit of a frump of a mouse. He's placidly happy as a watchmaker until an avatrix mouse mysteriously enters his life. I'm not usually a fan of rodents as protagonists, but I loved these stories.
Pictures of Hollis Woods, Patricia Reilly Giff. There's a mystery around why Hollis left her last foster home. The story unravels slowly as Hollis tries to keep life with her current increasinly senile foster mom together. Sweet sweet book.
Hoot, Carl Hiassen. The narrator, wallowing in his misery since his parents made him move, sees a barefoot boy running free outside his bus window. What follows is at typical Flordida-set Hiassen escapade the involves a land deal, environmental protection laws and ecoterrorism.
Silent to the Bone, E.L. Konigsberg. Why has Branwell stopped talking? What does it have to do with his infant sister, Nikki, who is currently in a coma. Who can get him to talk to tell us what happened? Griipping story by the woman who brought us the Mixed Up Files of Basil Frankweiller.
Not really mysteries, but fabulous stories that have been faves in the 5th/6th classrooms I've been in recently:
Twelve Again by Sue Corbett. The mom of the story leaves the house to escape her demanding children and husband. She is at her late mother's house when she wishes she were twelve again. Something happens and suddenly she is. A great story told in two voices.
Flipped, by Wendelin Van Draanen. Also told in two voices. Juliana has loved Bryce since she was 6. But he can't stand her and her weird ways. Until something happens and their perspectives on each other flip. A great story about tolerance.