Relying on the old Peterson Hawks of North America did not do much in the way of improving my raptor ID skills I'm afraid to admit. Two newer books which I highly recommend are Brian K. Wheeler's Raptors of Eastern North America and Jerry Ligouri's Hawks From Every Angle. Wheeler's book gives a wealth of information about each species. The maps are excellent and very specific. The photography is well done and the range of plumages, races and morphs for each species is comprehensive. The strength of Ligouri's book is in its specificity. It is designed to help observers ID hawks in flight, and goes about it in a very direct way. I especially like how he illustrates hawk ID pitfalls by putting the birds side by side in similar attitudes so the reader can see exactly what the difference is. Both books are from Princeton University Press.