A professional illustrator once gave me the advice that a good cover should sum up the contents of the book in one image =)
Well, I'm still far away from that.
The best covers I have ever seen are from @shazzbaa: Each of the Runewriters covers tells its own story <3
Source: http://shazzbaa.com/storeDetail.php?p=18
Go check out the corresponding website, the art is gorgeous and I learned so much by just looking at the pages! @_@
http://runewriters.com/
(Edit: Shazzbaa, if you're not ok with the image posted here, please let me know, I'll delete it immediately, okay?
)