My three-and-three-quarters old niece is mad for 'One Red Dot' and 'Blue Two' pop up books, the first of which she's loved since she was about two. They both feature the most amazing abstract pop-ups with a red dot or a blue two hidden within them, which are fun to look for. Cool for adults, too!
Quite good as well, I think, for teaching little ones to look after things carefully, as they have to be careful with the delicate pop-ups, as my niece has been.