The following links are of a producer stating back in july of 2016 that the movie is a "sequel" to the books. Also Stephen King posted a very telling (If you read the series) picture of what really to expect from the movie. If you have never read the series this will be just another story to you and hopefully you will like it. If you have read the series all the way through......SPOILERS....
You get to see the "Last" journey that Roland makes after being thrown through the door at the top of the Dark Tower. During the series, Roland tells the story of the Battle of Jericho Hill where his friends die. He gave Cuthbert the Horn of Eld and it stayed with Cuthbert after his death. Roland never retrieved it. When he is in the tower in the last book, he hears a voice tell him "It would have taken 3 seconds to pick it up" after that he reaches the door and is thrown back to what is essentially the beginning his journey. The book ends with the first line of the first book in the series. BUT this time he has the Horn of Eld with him.
So this movie or "story" of his journey will be essentially the same quest but with differences in it because the story has changed. He has a key element to his journey that he didn't have the first, countless, times he has done it. kind of like a "Egde of Tomorrow" trip. There is obviously a lot more to it but i distilled it down from 5000+ pages haha anyway hope this helped in anyway if you feel disappointed after having read the series and saw changes to the story. However there are somethings that still don't line up even with it being a new journey but thats the book to movie biz.
http://screenrant.com/dark-tower-movie-sequel-theory-books/
http://www.cinemablend.com/new/Stephen-King-May-Have-Just-Revealed-Major-Clue-About-Dark-Tower-131957.html