Under the Egg, by Laura Marx Fitzgerald

As Theo's grandfather lies dying he tells her to look "under the egg." This cryptic message actually makes sense to her as there is a prominent painting over the mantle of an egg that he painted.  So, she looks where he told her, checking every corner and crack around and behind the painting, but finds nothing.  That is, until she discovers that the painting covers a richer and far older artwork:  a painting that Theo comes to believe (on the basis of her strong familiarity with art) might be a lost work of Raphael.

But why would the family possess such a priceless work of art?  Where did it come from?  And does the fact that her grandfather used to work as a security guard at the Met mean that it might in fact have been stolen?  With the help of another girl and a motley collection of characters including an Indian hot nut vendor, a priest with an art history background, a burly librarian, an eccentric French tea seller, her absent-minded mother, and her backyard flock of chickens, Theo finds her grandfather was much more than she ever imagined.

A middle grade mystery with lots of interesting historical notes, most notably about the theft of art from the Jews during World War II.  There is plenty of lighter material on exotic teas, urban chicken farming, and (of course) art history.  In many ways, it's a bit of a heavy work for middle grade readers, but the sensibilities are all appropriate.  You won't find a romantic thread here, but the girls are resourceful and brave, the story educational, and the pace brisk.  I found it fun and enjoyable.