Peek-a-Who is a simple board book with cutouts so that the baby can look through and see part of the next page.
We actually bought it after Sadie repeatedly checked it out of the library between her first and second birthday. She loved it and it's become a family favorite because of the smiles it inspires in both girls (Sadie will still pause to hear me read it!).
At an age when Mae is quickly frustrated with books (she wants to turn the pages, but struggles with the motor skills required at this point) and wants to turn the pages before I finish reading anything more than a few words, Mae loves the wording (it's pretty much "Peeka" on one page and then another word like "choo choo!" on the next) and the Peek-boo theme!
(picture from amazon book page)