It's becoming harder and harder to judge a book by its cover. Because what I've noticed is that synopses are now often replaced by a list of awards/opinions from reviewers or other authors. Which, ok, I'm happy they enjoyed reading this book but
I'd like to know what it's about
But, yeah, as pretty much everyone here has said before me: title, cover, synopsis, first chapter. It's not a foolproof plan but it usually works. Sometimes my friends suggest books to me. I study literature and sometimes I like the author so much I decide to read more.