The Biology and External Morphology of Bees: With a Synopsis of the Genera of Northwestern AmericaBy W. P. Stephen, G.E. Bohart, and P.F. Torchio
Full Text here
An excellent resource with very detailed discussions of bee morphology, behavior, and biology---from identification to genus, to in-depth coverage of (a surprisingly diverse variety of) nest architectures. Very readable, even though largely written at a technical level. Many helpful diagrams throughout. In particular, nearly every couplet of the key to genera is accompanied by illustrations for clarifying the characters and terms being discussed.
I find this book to be a very interesting and helpful reference...only wish it included all of western (and not just north
western) America! Nevertheless, I think much of the utility and information in this book probably applies to areas beyond the northwest---especially to adjacent areas.