Now, there's no way I can get every classic of every genre that's influenced World of Warcraft into a list that would fit on this site. There are hundreds of potential books out there. So I'm just going to hit some highlights and let y'all go wild in the comments filling in the blanks.
J.R.R Tolkien's Lord of the Rings - Tolkien is the 10,000 lb gorilla in modern fantasy. If you're not influenced by him, you're a reaction to him. The reason there are multiple kinds of elves and dwarves running around Azeroth fighting orcs is because of Tolkien's impact on fantasy. If you want to get the tropes, you should probably read this.
Robert E. Howard's Conan and other stories - Howard is the other huge gorilla influencing modern fantasy. I mention Conan as his most famous creation, but there's just as much good stuff to be read by Howard that has nothing to do with the Cimmerian. His Solomon Kane, Kull, Bran Mak Morn, and his forays into horror and historical fiction all blazed from an imagination so incandescent that it burned the man himself out in a short amount of time. Be warned - most of this stuff was written for the pulps, and it has all the virtues and all the flaws of pulp fiction written in the 20's and 30's. It's often racist, sexist, and ranges wildly in quality.