Is it true that the movie is a sequel to the books?
Sort of...
SPOILER ALERT!!!!!!!!!!
The end of the book finds Roland opening the door at the top of the Tower...and finding the Man in Black fleeing across the desert (exactly the way the series opened.) It's a circle...it all plays out over and over again.
As I understand it, the movie is supposed to be a "new" beginning of the saga...after Roland has ascended the Tower once again.