"The Indian in the Cupboard" by Lynne Reid Banks

This is one of my absolute favorite books of all time.  I loved it as a kid and I love it now.  I hadn't read it in more than a decade, but I pulled it out to read aloud to my kids, and they loved it too, especially my daughters.

Omri's friend Patrick gives him a little plastic Indian for his birthday.  Omri's brother gives him an old cupboard he found in the alley.  Omri's mother finds him an old, old key that fits the lock on the cupboard, and he's delighted because now...