It depends what you really want, species-specific information, or a general book. Both books already mentioned are good, and there are a few others. Paul Johnsgard authored "Waterfowl of North America" in 1975, and "Waterfowl of the World" in 1978. If you're interested in Wood ducks, there's a book on wood ducks by Frank Bellrose. For stifftails, there's the book "Ruddy ducks and other stifftails" by Johnsgard and Carbonell. There are 2 books, one on non-breeding biology and 1 on breeding biology which are chapters by invited authors which are good literature reviews. I am currently reviewing "Waterfowl Ecology and Management" second edition by Baldassarre and Bolen. It looks to be a pretty good review (I like this edition better than the first). For species-specific information, I prefer the species accounts in "The Birds of North America." A complete set is outragously expensive, but single accounts are reasonable and excellent reviews of published information.
Clint