If you have a soft spot for old Penguin book covers, you'll enjoy these parodys by M. S. Corley. He has redesigned the seven Harry Potter books in the manner of vintage Penguins. Admittedly, my interest in these is also due to being a Harry Potter fan. The choice of subject matter and imagery is really appropriate to each book. Below are two favorites.
I prefer this stylistic treatment to the actual illustrations for Harry Potter. In their generality and reduction, these images are more universal and call forth larger themes in the books. The cover for The Chamber of Secrets shows the classic subjects of vanquisher and foe, knight and dragon, menace and victory. The Deathly Hallows cover is very psychological, addressing issues of fear and isolation. Who knew that we'd end up feeling sorry for "you know who." See the whole series.