From the Mixed-Up Files of Mrs. Basil E. Frankweiler by E.L. Konigsburg
From Simon & Schuster:
When suburban Claudia Kincaid decides to run away, she knows she doesn't just want to run from somewhere she wants to run to somewhere—to a place that is comfortable, beautiful, and preferably elegant. She chooses the Metropolitan Museum of Art in New York City. Knowing that her younger brother, Jamie, has money and thus can help her with the serious cash flow problem she invites him along.
Once settled into the museum, Claudia and Jamie, find themselves caught up in the mystery of an angel statue that the museum purchased at an auction for a bargain price of $250. The statue is possibly an early work of the Renaissance master Michelangelo, and therefore worth millions. Is it? Or isn't it? Claudia is determined to find out. This quest leads Claudia to Mrs. Basil E. Frankweiler, the remarkable old woman who sold the statue and to some equally remarkable discoveries about herself.