Thousands of books come out in the year. Most years I manage to read about a hundred. So, especially since I read in many different genres (all sorts of YA, some children's, and adult mainstream/speculative fiction/nonfiction/etc.), it can be hard figuring out what to spend my limited reading time on.
A few things matter a little:
Book description. If the story sounds really amazing, I'll look for more info. If the story sounds like 'not my thing at all', I may skim or skip further info. But there's a lot of space in between those extremes.
Marketing. If a book's selling really well, or being pitched really hard, and it's in a genre I *write*, I will probably take a look at it as market research if nothing else. However if I don't write in that genre it makes no difference.
Professional reviews. If I go to the Amazon.com page to read more about a book, I do check the Booklist/Publisher's Weekly/whatever other reviews. And if I like what they say, I'm more likely to pick the book up; if they make me nervous about whether I'd like the book, I'm less likely. But I have to have gone to that page for some reason in the first place, and that reason may very well outweigh reviews.
Cover. I don't know if I've ever picked up a book because of a cover. I have occasionally avoided books because the cover was awful, but only if I had little else to go by.
The one thing that matters a lot:
Reader recommendations. I think every single book I've picked up in the last few years I read because someone else said (to me directly, or in a post in their blog, or a comment on a message board, or wherever) that they really liked it. The more people I see saying they like a given book, the more likely I am to pick it up. The better I know the people recommending, and the closer their tastes are to mine, the fewer recommendations I need. And of course, if the story already sounds like something I'd enjoy, I don't need much encouragement--but if enough people talk about how wonderful a book is, even if it doesn't really sound like my thing, I will at least give it a try.
Mostly I think this is because if someone--who has nothing invested in a book, and doesn't have to review it for their job--liked it enough to want to tell people about it, well, that says quite a lot. And most of the people I talk with, or whose blogs or comments I read, are writers and/or avid readers who love books as much as I do. So I trust their judgment more than I trust sales figures.
Which means, from my perspective, the best thing you can do for an author you love is talk about their books. And the best thing you can do as an author is write books that people will love enough to talk about.
(If only it were as simple as that sentence makes it sound, no?)
How do the rest of you decide which of those thousands of books you will give a chance to?