Wulff,
nice photos. That's a female Northern Harrier. You can tell from the size, white-brown speckled plumage and long slender wings (also the face gives you a hint). It could perhaps be confused with an immature Northern goshawk, although, while it has the same colour scale, the im. goshawk gives a heavier chunkier impression. And of course, if you took these pics recently, there shouldn't be any immatures around this time of year.
Since I am not from North America, I dont't know much about the raptor litterature here, but comprehensive general guides, such as the "Sibley Guide to Birds", have paintings of several plumages and both sitting and flying birds. In general, however, I don't think Sibley's paintings of birds of prey are that great (he's very good with other species though). I usually use Sibley and Kaufmann's "Birds of North America" in combination, to get the advantages of both paintings and photos.