Book covers are my second most favorite art meets commerce products. Next only to album covers. If you want to while a small amount of time, and there is a book shop nearby go and browse. If you are a book loving person so much the better, but even if you are just an art or a design lover, go look at the covers.
A cover gives the book a small 5-10 second period to entice the readner’s curiousity. Once the reader picks up the book, then the onus of convincing the reader to buy the book goes to blurbs (by famous authors, newspapers etc), forward, preface, etc.
Anyway the book covers down below are vying for the best cover 2007. There are a lot more at The book design review page. You may want to go there and vote :).