if i'm browsing a bookstore? the font.

i've found that it tends to be the most accurate indicator for me on whether i'd enjoy the book or not - for example, garth nix's 'ragwitch' had an edition with a terrible generic horror cover, but the font was more of a dark fantasy.

i gave it a try, and it was fabulous. (...i should read it again, actually.)
if i'm looking for something specific, either new books by favorite authors or books recommended by favorite authors - i follow a lot of my faves on twitter.
...that is really only for fiction, though. :x nonfiction, i'd have to think~