Amazon has a 4 for 3 promotion on books right now. If you purchase 4 books that are part of the deal then the lowest price book that qualifies will be free. Here are a some options:

Harold and the Purple Crayon Board Book

First 100 Soft to Touch Words

Peek-A Who?

Potty (Leslie Patricelli board books)

Green Eggs and Ham (I Can Read It All by Myself Beginner Books)

But Not the Hippopotamus (Boynton on Board)

Fancy Nancy and the Late, Late, LATE Night