I go mostly for a book that's either strange, obscure, rare, or a classic. Rare as in not a lot of available copies. Obscure as in not well known, strange as in a little bit mystifying. And classics to me are mainly anything from the 20th century and before. However, I stay away from mass produced books, and contemporary authors who are only in it for the money. I don't bother with modern series books, just because it's a well known modern author really means nothing to me most of the time. But that's ok, we all have our preferences. Anyway, the cover of the book doesn't matter, usually for me it depends on the title. And I flip through the book and see if anything stands out to me. I look at the date the book was printed, what edition it is, and the table of contents. But anyway, all of this doesn't matter really. Over time you'll determine your own opinions that matter to you. Maybe you care more about a book's cover, I suppose that's understandable.