While reading the July/August issue of ForeWord magazine, I discovered a good number of great book covers. You can review these covers to help you design your own great covers.
I love the cover of this romantic thriller.
I also love the design of Mara Purl's Milford-Haven mysteries, like the one above.
Here are two poetry books with great covers.
Check out the book cover designers featured at http://www.bookmarket.com/101des.htm.