Terry Pratchett is, I think, pretty much unique amongst fantasy authors in that he is blessed with good cover art for his novels*.
First came the covers drawn by Josh Kirby. Even before I started reading the Discworld novels, I immediately recognised them on the bookstore shelves, courtesy of the glorious, garish pandemonium of the Josh Kirby covers. For example, you can see his cover for Guards! Guards! here. While the characters don’t look much like they do in the text, the cover is true to the book in the most important sense. It hints at the kind of reading experience you will, in fact, have: a bellyful of laughs.
Josh Kirby died in 2001, and in 2002 Paul Kidby took over the cover art with Night Watch. Kidby’s covers don’t have the same manic glee of their predecessors, but they depict the characters very well and retain the comedic touch that the subject matter needs. My favourite Kidby cover is Going Postal, which wonderfully spoofs the “barbarian hero posing atop a mound of bodies” (and I love the gothy Adora Bella Dearhart puffing away on her cigarette). And Kidby’s depiction of Sam Vimes as a Clint Eastwood-lookalike never fails to amuse me.
Truly, I would do both Pratchett and his cover-artists a great service to say I could judge the Discworld novels by their covers.
* At least in the UK and Australia; from what I’ve seen, the US covers aren’t quite so inspired.