The first thing that grabs my attention is the binding.
It has to have a certain color contrast to let you notice it out of the dozens next to it. A small picture of an axe, a wand, a dragon's head, anything to indicate the nature of the fantasy is great.
It also has to have a clear and visible title. If it's part of a series, it should say so as well. The Author's name will figure here, and it should be visible, since 9 times out of 10, a good author will pass the preliminary "binder" test.
Now that it won the pot and made me grab it, I immediately go for the synopsis. This should be linked to the main plot of the story. I can't forgive a book that has a synopsis about a side-quest or the first couple of chapters. I immediately ban the author whenever I get lured into buying books like that, even if the book turns out great. (Gee, this pizza is magnificent, why the hell does it say Lasagna on the box?)
Please, no reviews. The back cover filled with "The New York Times loves our nuts" or "Given the seal of excellence by the A.S.D.A.Q.G.W.L.K." turns me OFF buying the book.
And finally, if the binder is attractive, the synopsis is good, and the back cover isn't tooting its own horn, then I open up the book and look at the excerpt (There is one, right? Few books I've read didn't have one). In this part it's the writing style that gets me. If it lacks an excerpt, I'll just flip to a page in the middle and read 1 paragraph, even out of context, the writing style shows.
That, in a nutshell, is how I pick out books. For a new author, at least. If I'm anticipating the book I buy it blindfolded.
Let me summarize:
Judge a book by it's cover.
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