I think that this book may only have been sold at the gift shop in the Met during the late 1970s and early 1980s. My uncle sent me a copy, and I loved it until all the pages fell out.
The basic plotline is that a young boy named William is "dragged" to the Met by his parents. He finds a little blue hippo (who is in the collection) named William, and he and the hippo browse the Met together and learn about art. Apparently William (the hippo) is something of the children's mascot of the Met. It's a wonderful little story, but may no longer be commerically available.
This is "William" the hippo's website: