This is my three-year-old's favorite book. He likes to look for the skeletons he's seen at the Sam Noble Museum of Natural History, which he asks to go to every single day. (We have a pass, so we try to go at least once a week.)
It's a heavy book, with beautiful pictures. We started out just reading him the captions, but now that he's more familiar with it, we're actually talking about the pictures, and summarizing some of the text for him, too. The text is the kind of nonfiction that, if you just dip in at any point, you find yourself pulled in and next thing you know you've read the whole page and the next one. There are only two or three pages of text per group of pictures, and he usually steals the book from me if I stop talking long enough to actually read, so I only read it in bits and pieces. But I highly recommend it.