The Dark Tower series is my favorite series of books(Next to WoT of course). There are currently four books in the series:

The Gunslinger

The Drawing of the Three

The Waste Lands

Wizard and Glass

The books tell the story of Roland, Eddie, Sussana, Jake, and Oy. They must eventually reach the Dark Tower, which is the nexus of the universe. At the same time, the world is "moving on", and leaving the old way behind. No one is really safe during this time and the only way to keep the world in order is through the Dark Tower. There are obstacles in the way however. The Man in Black is the first enemy we meet. His master is the Ageless Stranger, also known as Randall Flagg(Please note that King has made it clear that in the world of the Tower all of his other books exist too, which I think is pretty cool) and the master of this Ageless stranger is the Beast, who is a direct agent of the Tower. I really suggest that you check these books out. I picked up the Wasteland one day after school whilst extremely bored, and I didn't put it down for about 100 pages.