Searching for Yesterday

Commended for the 2009 Best Books for Kids & Teens
Annie Berkley, a schoolmate of Shelby's who has appeared in previous books, lives in a foster home and has become depressed. Shelby is concerned and eventually wins over Annie's trust in order to find out what's bothering her.
Annie's mother disappeared several years ago, leaving with her boyfriend when Annie was 8. she was never in touch with her daughter again. But now, the boyfriend has returned, saying that he lost touch with the mother and doesn't know what happened to her.
Shelby suspects that something more sinister is going on, and as she digs through clues, discovers that the boyfriend knows far more about the mother's whereabouts than he's letting on. But the attempt to uncover secrets puts Shelby and Annie's lives in danger.