Steven King calls this "a sequel"
This movie is not Book One of the series. Apparently it is a re-worked sequel for those who didn't like the way the books ended. This movie starts somewhere in the middle of the books; that's the reason for all the minor characters.
http://www.cinemablend.com/new/Stephen-King-May-Have-Just-Revealed-Major-Clue-About-Dark-Tower-131957.html
http://www.cheatsheet.com/entertainment/the-dark-tower-movie-know-far.html/?a=viewall