I like to think of Tomoko Fuse as a mad genius when it comes to origami designs. Especially the hard stuff, you know the massive modular origami that requires multiple sheets manipulated into crazy geometric designs.
Unlike some books on modular origami, this does not just toss you in head first to a dodecahedron and expects you to perfect fold that make absolutely no sense. It gently guides you through units ranging from simple to complex, it is very much so a guide.
This book is considered by many to be the classic book on unit origami, and I tend to agree. I have (thanks to a many well stocked library systems) read many books on origami, and there are only a few I thought worthy of my personal collection. Unit Origami is, in my opinion, THE book for making the transition from single unit pieces to multiple unit pieces.