Ummm . . . This guy knows about publishing, but he doesn't know anything about marketing.

Without knowing as much as possible *who* is going to read a particular book, buy a specific thing, service, etc. It's impossible to know where and how to advertise effectively.

Unlike nonfiction books, fiction/SF/Fantasy, etc. books can be very hard to categorize and as you pointed out, it's very difficult to figure out what will sell. Publishers do promote new books when they know there's a large (and quantifiable number) of people who will buy a book because of the *author*, but not necessarily because of content (unless it's a really hot topic at a particular time).

My two cents :-)