I like the Kaufman Focus Guides "Butterflies of North America". The authors are Jim P. Brock & Kenn Kaufman. The book is organized by family and shows location maps and gives distinguishing features to tell species apart from one another. It also will list habitat, food plant and other basic information.
Thanks a lot, guys! That was for a chinese friend of mine who’ll soon study in the US, and after spending the summer with me collecting he really wants to start collecting too, so that guide will be a good start for him.