For wonderful examples of environmental portraiture "Arnold Newman" by Philip Brookman (Taschen) is a great inspiration. For more of a how to I recently stumbled across "The Photographic Portrait" by Robin Gillanders, which has some great reminders of basic portrait techniques as well as more in depth information.
I always thought Newman's "One Minds Eye" showed what a master he really was... and he is considered one of the great masters of "existing" light photography.
He told me he always called it "existing" light instead of "available" light because as his level all the lights he would ever want were "available".
If you're looking for something a little more National Geographic you can always check out David Allan Harvey, Bill Allard or Steve McCurry. McCurry is considered one of the great NG portrait photographers..... and I always felt fortunate to spend a week with him years ago at a workshop.
Go to the Photography section of a large library and look for books by these photographers, or the specific books mentioned. Each is a different kind of environmental portrait photographer.
W. Eugene Smith, An Aperture Monograph
Thomas Roma: Enduring Justice
Tice, George A.
Fields of Peace, A Pennsylvania German Album