The film is both an adaptation of, and sequel to, King's books* (see below, but SPOILERS). The plan is for this to be a multi-film project, with the first film drawing on elements from the first three novels in the book series. There will also be a TV series exploring Roland's backstory as revealed in the fourth novel, with Elba appearing in framing sequences for the flashbacks and a younger actor playing the teenage Roland.
The film is directed by Nikolaj Arcel (A Royal Affair) and also stars Tom Taylor, Katheryn Winnick and Jackie Earle Haley. It will be released on 28 July. The TV series will air in 2018.
SPOILERS AFTER THE JUMP
* The story of The Dark Tower forms a time loop, with the novels ending at the very moment they begin, with Roland Deschain beginning his search for the Man in Black. However, things can change with each cycle. At the end of the novels Roland gains control of a powerful artefact, the Horn of Eld, which he will be carrying in the films. As such, the movies form a continuation of the story in the novels as well as adapting them (because many of the same storylines and characters from the novels will be reprised, in a different form).