Also, is all of this from one of Lovecraft's books or is it spread across multiple titles?

The Cthulhu Mythos is a body of work that spans several books, includes many gods and locations and has been continued by authors since Lovecraft. Lovecraft is generally considered to be the originator but even his books were derivative of ancient Greek myths and other legends (like so much fantasy literature is).

Unlike in Warcraft humanity (especially individuals) were completely irrelevant, which is one of the more unique aspects of his work. As most fiction, even heady fantasy and science fiction of his time were at their core stories about people or individuals and how they face adversity. This is a reason why I find the connection between Lovecraftian Mythos and Warcraft Lore so interesting. They put the player at the center of this huge conflict which has absolutely nothing to do with them (but maybe they don't want us to know that).

Hey now there's another interesting Tin Foil Hat idea, Warcraft, It Ain't About You.