Well, firstly I look at the subject or genre I'm interested in reading.
Covers might catch my eye, but if the blurb on the back doesn't match the type of stuff I like to read, the book could have bells and whistles and flashing red lights on it and I still wouldn't buy it.
Then, I have to admit, I look to see if it's part of a series or is standalone. Because I just am not patient enough to buy one part of a series and wait for the rest to come out (when our store probably won't even pick them all up). Which means that, although I write almost primarily fantasy, and write almost primarily series, I read woefully little of either.
If all the books of the series are there and look interesting then I'm all for it! But that usually doesn't happen, of course.
But I confess I read very little fiction because there just isn't much fantasy out there that's really original anymore (i. e., that doesn't feature boy wizards or elves or vampires or something). I write the kind of stuff I'd love to find on shelves.
Finally I browse a few pages and see if the writing style draws me in or not. I tried this with the "Twilight" book despite it being about vampires (I thought, they have all volumes of the series on the shelf, people rave about it, maybe it's good?). I did not like the style, so put it back.
Such is life.
That's how I choose books. Even online. I adore being able to browse the tables of contents and back covers of some books at Amazon.
Also do you find yourself constantly trying to fix what's in a book at times? I know I do a little lol.
I never used to, but now I do, unfortunately a lot. One example, I'm of the "Prologues are part of the story! Read them, damn it!" school, yet with the book I'm reading right now, I found myself thinking, "Ah, the writer obviously used this actiony prologue to cover up the fact that Chapter 1 is rather slow to start the story." Then realized that I've done the same thing myself. But I'm not changing my story, darn it, I'll just make it less obvious!