We've been receiving and reading a few fun science books lately from a series called See How They Grow. So far we have the Frog, Calf, Owl and Penguin books, all from PaperBackSwap.com.
Each page shows a few pictures of the animal at a specified age, progressing with time through the book. The frog one is particulary interesting as it shows the tadpole changing into a frog. I think First Son could really understand and see the different stages of growth. (I'm hoping we can find a frog outside soon so I can bring up this book in casual conversation.) The books have just a few words per page, so they're great for early readers (though we don't have any of those in the house yet).
The calf one depicts a dairy calf, so no worries about heading off to slaughter. I guess that's a part of farm life most people don't want to share while snuggling on the sofa with the kids. (As a side note, I was surprised and oddly amused to learn recently that E.B. White promptly slaughtered his pig after writing Charlotte's Web. Come to think of it, I guess that's a book we'll probably avoid in the fall if we do decide to raise a couple of pigs next year.)
Now, the warning: The newer paperback versions come with stickers. If your kids are anything like mine, you will quickly find them throughout the house.
Oh well, at least there's only one page of stickers per book, and the kids did enjoy them!