Critical Mass, the blog of the National Book Critics Circle, has an interesting interview with Oscar Villalon, the editor of the San Francisco Chronicle's book review section.
Q: Are there any rules that guide how you sort through and pick the ones you'll review?
A: Self-published and vanity press stuff, of course, always gets tossed immediately. If a book is regional and its region has nothing to do with the Bay Area, that gets tossed. The first things I look at are: How pertinent is it to the region, and how pertinent would it be to our readers? We don't review romance novels. We do genres -- SciFi, mysteries and thrillers -- in monthly roundup columns, so those get set aside for that. Everything else I pretty much hang on to and flip through. Of course that's the other big chunk of my time -- going through those galleys and at least reading a little bit into them.
Choosing which books to review is completely subjective. You look at the catalogs and get a sense of what looks interesting, who you like. It's pretty much the same way you decide what you'll cover in a newspaper: You just go by your instincts. Clearly you pay attention to what's in the news and which books could be related to that, but for the most part, it's about picking books you would like to read. You have to trust your instincts and hope readers like to read what you're interested in. If your interests are too esoteric, you're going to alienate a lot of people, and if your interests are too broad, you're going to pique very few people. It's got to be somewhere in the middle.