Matt,
Any Caldacott winner is always a good choice (this is a special award for children's books). The classics are always a good idea: Where the Wild Things Are, Blueberry's for Sal, Make Way for Ducklings, etc. Also, the Dr Seuse (?) books. I always enjoyed Are You My Mother?, One Fish, Two Fish, Red Fish, Blue Fish, Green Eggs and Ham, etc. The Curious George Books are good also. Good Luck. Books are a great way to help children learn. You can also get books pertaining to subjects they are interested in, like dinosaurs, drawing, fashion, etc.