The short list:

The Joy of Photographing People, Kodak

Robert Caputo, National Geographic Field Guide: People & Portraits

The Portrait: Professional Techniques and Practices, Kodak Workshop Series

Steve Sint, Digital Portrait Photography: Art, Business, and Style

Christopher Grey, Master Lighting Guide for Portrait Photographers

Jeff Smith, Posing for Portrait Photography: A Head-to-Toe Guide

You can learn more about these books, and buy them, by going here.