I can't recommend Photography After Frank highly enough. Philip Gefter was a photography critic at The New York Times for many years and was also the art director responsible for picking the main front-page image for the paper everyday; he has an extraordinary knowledge of photography from a multitude of angles. I've been plodding along slowly with this book because I'm trying to write down the name of every photographer he mentions and research the ones I don't know. Read a good review at The Year In Pictures and buy it directly from Aperture.
Susan Sontag's On Photography is kind of the classic in the field and I'm just now getting around to reading it. It's a little stinging in parts because she makes some sharp psychological observations about this desire/need that certain people (ahem) have to, uh, document everything. That she was writing long before cell phones, digital photography, and the web makes them all the more true now. Still, I'm finding that it makes me look at photography in different ways and I appreciate that.I wonder if all art directors secretly want to be photographers.