OK, for starters, Light: Science and Magic is made of awesome. It's simple, it's clear, it's well-organized and it covers a lot of ground.
I picked up a pair of books on portraiture. While nice, they're not of the same caliber.
I bought Erin Manning's Portrait and Candid Photography Photo Workshop (from the Photo Workshop series) based on a short video promoting it. This video:
It's a nice little video. It also covers the best material in the book.
Portrait and Candid Photography Photo Workshop is pretty well organized. It wastes a fair number of pages on dated information about equipment. Still, it's pretty good on composition, demonstrates some good quickie problem-solving techniques and keeps hammering home the two real basics: keep your subject comfortable and watch your backgrounds like a hawk.
I'm not sure what drew me to The Perfect Portrait Guide: How to Photograph People but it may have been the odd composition on the cover photo. What I got was a "cleverly" designed book with only the most rudimentary organization.
That's not to say it's bad. It's just clumsy.
It doesn't really waste many pages on equipment. That's a good thing, the examples are shot by a wide range of portrait photographers using their favorite equipment, whether it's 35mm, medium-format or large-format. It could be even more out-of-date than the Manning book.
That's the great thing, though. There are many pictures by many photographers, some with in-depth analysis of the shot, some with just a cursory notes, some with nothing but the shutter speed, aperture, lens length and film used (yes, film).
It's not a good instructional book on how to shoot portraits. It's a good instructional book on how to look at portraits in any media and figure out how and why the composition and lighting works. If you already know something about composition and lighting, it's probably the better buy.