Kathleen O'Dell's The Aviary
Wonderful, wonderful MG book about a little girl growing up in a big ol' falling apart mansion--her mother is the caretaker for the elderly woman who lives there, the widow of a famous magician, a kindly woman who loves her roses and her birds. When the girl hears a story about the woman's baby son being kidnapped many years ago, she decides to solve the mystery (with the help of a new friend). O'Dell doesn't drag out plot points that are easy for an older reader to figure out; the pacing in general is pretty strong. Great characters (especially the birds) and great resolution. A.