The cover doesn't interest me most of the time, i read the summaries and book reviews and/or sample sites and make my decision based on that.

I first notice the book's cover then I read the book's summary

I typically will look at covers that catch my eye(sorry no offense...I know a book isn't judged by its cover) and the title of the book. Then I'll read the summary. If it sounds like something i'm interested in I'll scan the book to my phone on an app called goodreads to mark it. I do prefer hardcover books, but as I am now on a budget as a college student I got used to paperbacks and wait for the book to come out in that form.

The summary and the book's title. The cover is also a factor.

I usually decide based on reviews in my favourite SF/F magazines (as most of what I read is science fiction/fantasy/horror). Once in a while I'm recommended something by a friend, and if I pick a book without any prior information, I go by the summary and maybe skimming first few pages.

The title and cover-art will get me to pick a book up, but the summary is what gets me to take it home. Unless it gets a very strong recommendation, then I might just read it anyway.

I use a whole slew of things to pick out books.

When in a bookstore, titles attract me first, and then summary. I can't lie and say covers don't play a part, but I try not to judge quality on that, because I've read some really great books with really terrible covers. And if it's by an author I've enjoyed previously I'm more likely to pick it up right away, no questions. Unless the premise is just super unappealing.

Price plays a part too. I'm less likely to buy a book I don't know more than the summary about if it's more expensive. If I'm not completely sure I'll love it, I tend to research it and read non-spoiler reviews.

I usually go for recommended books, but sometimes I read a book if its summary is interesting. The author also plays a small part

It's a toss up between recommendations from the book store, the cover and the summary.
I have to say though if a cover doesn't grab me, chances are, I won't pick it up.

The cover will make me pick a book up, the summary/blurb will make me put it back down again. I'm an avid reader so if the summary is catchy, I'll then open it; but really I've found I can spend 2 hours in a bookstore reading 2 lines of a blurb and putting it back immediately. I think I've just read too much bad story and now I get alarm bells when I see something that leads down the track to badstory. It's a pity, cause some otherwise great books that I read on recommendation, I wouldn't have given a second thought in a bookstore cause of bad blurbs. So I guess, the blurb/summary is the real seller for me.