Take Karen Katz's fantastic illustrations, combine them with a simple lift-the-flap story, and you've got a set that should provide hours of fun for your baby.
This set includes 3 of her more popular titles. Belly Button and Toes both talk about different body parts, while the third book is a hide-and-seek game between Mommy and baby. The first 2 are useful in introducing children to names for their body parts, and all 3 will amuse even younger babies simply because of the drawings. Our 7-month-old is too young to grasp the concept of noses, tummies, etc., but she loves to pull the flaps and to make excited noises when she sees the illustrations of the babies.
You'd think that these might wear out their welcome quickly, but that hasn't been my personal experience. I've found that kids will continue going back to these ones over and over again. An inexpensive and worthwhile purchase.