There are a lot of resources out there for aspiring cover designers, or for anyone who wants to understand composition and design choices a little better. A few of my favourites:
Cover Design Studio is one of the most comprehensive out there, with designs, tutorials, and a blog explaining the finer points of cover composition. If you’re looking for a detailed examination of how the rule of thirds works, or an overview of the kind of typefaces one should look at, you could do worse than noodle around on here for a few hours.
The Book Designer is another site with a ton of practical advice on what makes a book stand out in the marketplace – check out their discussion of what makes or breaks a Kindle ebook, for example, or this love letter to the typeface Bembo.
Finally, there’s Creativindie, a blog about taking the “starving” out of “starving artist”, which takes the common-sensical (not to say cynical) approach to issues like cover design. I especially recommend articles like Book Cover Clichés: Why Using Them Will Actually Help You Sell More Books and, for the typeface geeks among us, an epic list of aesthetically-pleasing fonts, by genre.