Useful Books and Web Sites for Bee Watchers
There may be better resources out there, but I'm limiting this list to what I've used and liked: two general field guides, one good general reference book, one great book on honeybees, and two excellent Web sites.
Good Field Guides
1. Kaufman Field Guide to Insects of North America by Eric R. Eaton (My favorite field guide for figuring out what I was watching at the sunflowers, or at any flower)
2. The Audubon Society Field Guide to North American Insects and Spiders by Lorus and Margery Milne (Sharp photos of the major types of bees; contains a few mistakes but not big ones)
Good General Reference Book for East of the Mississippi
Insects: Their Natural History and Diversity by Stephen A. Marshall (Detailed and easy-to-read introductions to all the types of insects found in eastern North America, with loads of good photos)
Great Intro to Honeybees
The Dancing Bees by Karl von Frisch (Everything you wanted to know about the unusual life of honeybees, and lots more you didn't know you wanted to know, by the man who discovered the language of honeybees. Written for people like me who knew nothing about insects. Also has a brief chapter on some of the solitary bees you might find in your back yard and how their life-styles are different from honeybees'.)
Excellent Web Sites for IDing Bees and Other Insects
1) www.BugGuide.net (Want to know what kind of bee you just photographed? Look through the online photos at BugGuide. If you still can't figure out what it is, sign in and post your photo to "ID Request" at BugGuide. One of the many experts who volunteer there will look at your photo and will tell you what you have -- for free.)
2) www.DiscoverLife.org (Already mentioned on GreatSunflower.org as a major resource. Extremely detailed; you'll need to record very accurate observations -- sometimes requiring a microscope -- if you want to use their charts to ID an insect down to the genus and species level. More for the professional than for inexpert amateurs like me.)