Firefly Books publishes "Waterfowl of Eastern North America," which is an illustrated guide to ducks and other waterfowl to the west of the Continental Divide. Popular generic field guides, like the "Peterson Field Guide to Birds of Western North America" and its eastern counterpart contain most major American waterfowl.
The "National Audubon Society Field Guide to Birds," also split into an Eastern region and a Western region, is another popular birding guide that contains many species of North American waterfowl.
Ducks Unlimited has a free online guide to ducks and other waterfowl. The guide include pictures, sound clips of the calls of individual species and detailed descriptions on appearance, breeding, population and migration.