Origami Zoo by Robert J. Lang, Stephen Weiss (1990, Paperback)

This is one of the first origami books that I ever learned from, but I can only apprciate it now that I am a more experienced folder. It has a very nice selection of models (all animals, obviously) and has difficulties ranging from easy to VERY hard. One thing to note is that most of the models in this book cannot be folded without some previous experience, for the difficulty increases quickly. The pro, however, is that experienced folders will have a very challenging and rewarding origami experience; the models are usually 3-D and have very clever and thoughtful folds. A must-have for any origami enthusiast.