My son is 3.5 years old. Here are some of the books he loves.

The Magic School Bus series. For example:

http://www.amazon.com/Magic-School-Bus-Ocean-Floor/dp/059041...

If I Built a Car (a great book on design)

http://www.amazon.com/gp/product/0142408255/ref=oss_product

Manfish: A Story of Jacques Cousteau

http://www.amazon.com/gp/product/0811860639/ref=oss_product