The Mysterious Edge of the Heroic World (by E. L. Konigsburg)

Mrs. Zender, about to move into a retirement community, has hired William's mother, an estate liquidator. Amadeo, fascinated by both of them, and hoping one day to make an important discovery, volunteers to help with sorting through the mansion. Meanwhile, his godfather, Peter, a museum curator, is preparing for an exhibition of art considered degenerate by the Nazis. This gives Amadeo the background to recognize a mystery both artistic and historical among the fragments of Mrs. Zender's strange life.

All the classic E.L. Konigsburg themes are here: precociously gifted children, mystery, art history, a brilliantly flamboyant and cantankerous old woman, and deep friendship. Her story is gorgeous and fascinating, but subtle; she is not one to spell everything out for her readers, preferring instead to leave arch and tantalizing hints so that they can figure things out for themselves.

This is not for everyone, or even for most. Konigsburg expects her experienced readers to be equally clever as her gifted characters, to be comfortable with ambiguity and sophisticated vocabulary (or at least willing to look things up), and to be willing to think. If that describes your child, he or she may love this. If it doesn't, but you wish it did, a better place to start would be the author's more accessible From the Mixed Up Files of Mrs. Basil E. Frankweiler.