These are fun subjects, I chase them around too!
I particularly like the lift-off/landing shot with the wing blur and the mandible either extending ready for action or not yet folded up to the ventral surface, depending on whether bee is landing or lifting off. And the pollen sac gives a lovely touch of color.
http://www.pollinator.org/books.htm bumblebee books, eastern US and western US species
http://www.pollinatorsnativeplants.com/ book on pollinators and native plants interactions, quite good short chapter on basics of pollination ecology, terms, visual glossary of plant and insect parts; bulk of book is organised by plant species, listing pollinators and parasites, lots of photos. I recommend it - I needed something basic. Coverage is Upper Midwest and prairie states, also applicable to my state's prairies and glades (Missouri).
