By E. L. Konigsburg
A sister and brother run away to live in the Metropolitan Museum of Art and end up chasing down the mystery of a small marble angel.
Eleven-year-old Claudia has decided she is done being unappreciated, so she runs away. Not to anywhere cold or uncomfortable, though. She picks the Metropolitan Museum of Art, and she brings her younger brother Jamie along mostly because he has saved up the money. They sleep in a fancy old bed, wash up in the fountain and pocket the coins people have tossed in, and hide in the bathrooms when the guards come through at closing. Then the museum puts a small marble angel on display, and word gets around that it might be a lost Michelangelo. Claudia decides she can't go home until she knows who really carved it, and the trail eventually leads them to the statue's former owner, the very particular Mrs. Basil E. Frankweiler.
This is a mystery with nothing scary in it. No magic, no villain, nobody in danger. The pull is a great secret (two children living in a museum) and a real puzzle (who carved the angel), and the fun is watching how clever they have to be: the budgeting, the planning, the library research. Claudia and Jamie argue about almost everything the way actual siblings do, so it's funny as often as it's suspenseful. If your child likes to work things out but wants a book that won't frighten them, this is a good match.