This small, compact Peterson field guide has a wealth of identification information for North American raptors. Besides the typical Peterson "arrow" technique for noting key identification marks, it also includes many flight photographs in the end papers of the guide.
I have the first edition (1987) which is okay, but not current enough. It also has black and white photos that aren't very helpful. It's the only hawk-specific guide I own, though, so I do use it sometimes. I'm just not much of a hawk watcher, so I haven't bothered to get a better hawk guide.