Author, Author, Author
I believe in the "auteur" theory of book-buying. Since the author serves as the the rough equivalent of a director, screenwriter and crew of his/her creation, this is about the only criterion I use to choose a book that I will purchase. The format of the book is immaterial. Even early reviews are irrelevant. Thus, if Raymond Chandler, Dashiell Hammett, Herman Melville, or Theodore Sturgeon were still alive, I'd buy their books without hesitation. With living authors, I'll still order certain books in advance, but it's also sensible to browse through a novel in a bookstore, just to make sure the subject interests you enough to spend several hours reading the book from start to end. Sadly, some books just don't hack it with this test. How many pages does it take for me to decide that a book is just not worth perusing/pursuing any further? Three pages. I must fall for a book, hook, line and sinker. Otherwise, it's CUT!