If I'm walking into a bookstore with no particular book in mind, then it usually depends on how well one can attract my attention. I look for how unique the image on the cover is or how unique the title is. I pick it up, read the summary on the back or inside cover, then decide from there. I decide taking into account my interests and if the subject of the book would adhere to any of them.
Oh and for manga (the anime comic business), almost all of my decision depends on the quality of the artwork. Then it's the summary.