The Dark Tower: 15 Easter Eggs You Probably Missed
13. Misery
The fictitious novel which served as the catalyst for Paul Sheldon's nightmare ordeal in Misery can be seen propped up on a console at the Man in Black's base of operations.
Misery's Child is the book that sent Kathy Bates's Annie Wilkes full psycho. She didn't take the revelation that it would kill off the titular protagonist too well, you could say.
Presumably one of the Man in Black's minions left it lying in round for him, just in case he gets time for a spot of light reading when he's not trying to vanqiush all of reality.