If the "Library Bag" feature has nearly disappeared of late, blame my lack of good picks. The only great title we've checked out in a while is a small children's art book called Hello, Fruit Face! The Paintings of Guiseppe Arcimboldo.
His paintings depict the four seasons and other natural phenomena as incredibly realistic figures. Your children will be fascinated! amazed! repulsed! We studied the details for hours. The text gives easy-to-understand historical background and great discussion starters.
Oh, and if you ever see a secondhand copy of Hello, Fruit Face? Snatch it up. Amazon's start at $120. Used.