This is the flipside of book marketing. If you understand why people read what they read, then you have a better handle on where and how to market.
For me, it looks somewhat like this:
I buy my first book by a particular author book because I have heard of the author, and I’ve heard that he/she writes what I enjoy, and I’ve heard that other people have enjoyed this author’s books. This ‘hearing’ of an author happens informally, usually in cyberspace. The author might be part of a forum, the author might write an interesting blog or I might have attended panels by the author at a con, or other such interactions.
I rarely (almost never) pay attention to reviews (don’t read them). I target the books I buy, regardless of the book’s ranking on Amazon or elsewhere.
What about you?