Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
The Biology and External Morphology of Bees: With a Synopsis of the Genera of Northwestern America
By W. P. Stephen, G.E. Bohart, and P.F. Torchio
Agricultural Experiment Station, Oregon State University-Corvallis, 1969
Cite: 307295 with citation markup [cite:307295]
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 northwestern) America! Nevertheless, I think much of the utility and information in this book probably applies to areas beyond the northwest---especially to adjacent areas.