I picked up the WoW RPG a few years back before I knew anything about D&D/d20 and am just now getting around to checking out D&D and am wondering where this book fits into the d20 versions?
World of Warcraft Second Revision (which is the edition that I have myself) is compatible with 3e/3.5e d20.
I've actually used a few ideas/concepts/classes from those sourcebooks in my 3.5e D&D game.
3.5 is the version it was officially published with. It therefore has a degree of backwards compatibility with Pathfinder.
There is also an ongoing fan project to convert Warcraft into 5e--Here is a link. https://goo.gl/cmCypo