National Gallery et al. Renaissance Faces: Van Eyck to Titian. 2008.
This fabulous book delves into the processes and procedures behind the portraits we know and love. It has several good essays recounting the interactions between models and artists in the renaissance; the problems and rewards have not changed in centuries. One of the best illustrations compares two artworks--one sketch of Charles V Hapsburg as he truly was, against a painting of him politely idealized by Titian. The difference is startling, and the text delves into the practice of idealizing the sitter. This is a great book, not only for the pleasure of looking at the interesting faces, but also for the careful research done by the authors.