When I first began keeping bees (almost ten years ago) I also began taking more notice of the diversity of native bees that visited our yard, especially the extremely photogenic bombus. What was frustrating is that it was very difficult to ID the bees, and there was no North American guide to bumblebees available.
Xerces society offers handy pocket ID charts as well as these information packed guides for Western and Eastern United States (free pdf downloads).
News Flash: Princeton Press just released the first comprehensive guide to all 46 North American bumble bees. It's jam-packed with color photos (of live bees too! not just bees on pins), color pattern variation diagrams, distribution maps, taxonomy, and latest molecular research. The introduction includes photos of insects who mimic bumble bees, habitat and foraging facts, and how to use the guide.
Bumble Bees of North America is a wonderful resource for all backyard beepeekers and gardeners.