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
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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