The War of the Ancients, consisting of The Well of Eternity, The Demon Soul, and The Sundering details the first invasion of the Burning Legion in Azeroth. Brought to Azeroth by the hubris of the Night Elves, the mortal races of Azeroth must stand together under the leadership of a few familiar faces to prevent Sargeras from entering their world.
Having some previous knowledge of the warcraft universe will make this read more enjoyable, but don't let that hold you back. You might miss a reference or two from another book, but it is still worthwhile. I was unable to put these books down when I started to read them, it consumed me.
tl;dr: Awesome books, go read.
