grey318

You probably saw at least one of their work. Specialized in book covers, grey318 is a London-based studio working mainly for UK and US publishers. The most known covers are probably the Nick Hornby ones, for Penguin.
In their work we can see an accurate choice of font and a special translation from words (the book) and the images (the cover).

So now you can judge a book by its cover. ;)

Link: grey318