While reading Zoo-logy by Joelle Jolivet today, my daughter pointed out the caribou. She loves animals and books, and this has instantly become a timeless piece in our children's library.
In this gigantic book (it's big!), animals are categorized by where they live, colours, etc. And with more than 300 animals, there's an index in the back describing each one. I like that it's a book my kids can grow with from the familiar zebra to the unknown capybara.