I have a couple of recommendations:
--People Photography by Chris Orwig. Really more of a holistic look at shooting people than a technical one about setups, lighting, and using your camera.
--The Complete Portrait Manual, by Popular Photography. Pretty much the complete opposite of Orwig as I recall (it's been a while since I looked at it). Basically it's a bunch of tips.
--Complete Guide to Photographing People, by John Hedgecoe. Hedgecoe has forgotten more than most of us know. Good solid piece on shooting people. He also has another book that is on shooting nudes and portraits.
I also find it invaluable to look at the work of great portrait photographers. So Anne Leibovitz, Steve McCurry, Dorothea Lange, Yousef Karsh, Richard Avedon, and Herb Ritts are a diverse but accomplished mix of photographers all known for their portraiture.