The Publishing Room
Maintained by Raymond Long. Disclaimer: any information on this website could be wrong.

What makes a print book sell? I think the main factors are:
1. Physical placement - the publisher pays bookselling outlets to place the book where it's most prominently displayed. (I used to know someone who did sales analysis for Random House, and around 2006 he told me this was the number one factor.)
2. Author recognition - the author is already well-known as an author, or in some other field.
These things tend to go to authors who are already big names. As a newcomer it's hard to break into the market.

Analysing the book market
Booksellers Association market reports (mainly UK and Ireland).
Publishers Association statistics.

Something I can do for myself is to see what kind of books are selling best in the genre I want to write. I do this by looking for the bestselling books in the appropriate categories on the big bookseller websites. If you do this once a week and read the blurb for the top-selling ten books in the genre, you soon get a feeling for what's selling well now on the internet. Of course, what sells well in physical bookselling shops may not be the same. But there's usually a lot of overlap.
Print book sellers
Barnes and Noble print books.
Amazon (US) print books.
Amazon (UK) print books.
Multiformat ebook sellers
Barnes and Noble ebooks.
Fictionwise ebooks.
Proprietary format ebook sellers
Amazon Kindle ebooks.
Sony ebooks.

Award winners
Another useful form of analysis is reading award winning books in your chosen genre.
Dagger Awards awarded by the Crime Writers' Association - detective and crime fiction.
Hugo Awards awarded by the World Science Fiction Society - science fiction.
Nebula Awards awarded by the Science Fiction and Fantasy Writers of America - science fiction and fantasy.
World Fantasy Awards awarded by the World Fantasy Convention - fantasy.