Apparently, there are / were plans to make a trilogy (+ TV series?) of the Dark Tower books by Stephen King. I will now tell you how to adapt the Dark Tower books to the screen effectively. Any deviations from this plan will be atrocious, I assure you.
First, a caveat: I have not actually finished the series. I read the first book many many times, and it is tied very closely in my memory to various experiences during my formative years even my love of reading itself. I read the second and third books hungrily and was quite excited by them, but did not resume consumption of the remainder until some time after the fourth was published and the series resumed.
After the third book, my experience was with audio books. I listened through about a third into the final book. I still have the audio book on my computer, but doubt I will ever be able to stomach going back to it.
The success of the first Dark Tower stories lies entirely in their imagery and atmosphere, the old west, the Beatles, dark magic—these elements seamlessly blend into an organic whole which evokes a magical picture in the reader's mind.
By the last few books in the series, however, Stephen King had fallen in love with his characters. For a storyteller, this is the death of their craft, in particular, the death of arc and character—after he fell in love with his characters, he was scared to kill them off, and, correspondingly, unable to produce legitimate tension in the story; after he fell in love with his characters, he was unable to make them cruel, and, consequently, Roland the anti-hero became Roland the kind and wise uncle whose supposed willingness to sacrifice his loved ones for the quest was given lip-service, but not realized in any convincing or tension-inducing way.
Furthermore, King's near-death experience induced him to force a post-modern turn on the series, emphasizing a multiverse perspective, fatalism, and introducing himself as a character. Fatalism kills suspense, it kills character, it removes the incentive for motivation, which drives both character development and plot—fatalism is the the bane of modern genre storytelling.
So, fuck the end to the last Dark Tower book: King, you had your chance and punted—if a Dark Tower movie adaptation is going to be any good whatsoever, it will have to follow the first rule of adaptations in a very serious way and feel free to violate the source material.
Usually this means feeling free to replace features of the source material inappropriate for the relevant new medium. For example, the Harry Potter films erred on the side of including incidents from the books rather than developing characters in the spirit of the books. Less time spent on including all the scenes from the first books in early movies, and more time spent on showing us in a manner unique to the medium of cinema the same character features that were demonstrated in the books in a manner unique to the medium of text would have greatly improved the quality and efficacy of the early movies. (I refrain from commenting on later ones as I am unqualified.)
So, the rules for adapting the Dark Tower books to the screen effectively are
1. Ignore the NYC plotline; ignore the multiverse aspect; ignore everything that is self-referential.
If the multiverse is included, everything will become convoluted, in-joke, inaccessible, and directed only at the fans or King himself. If it is abandoned, then there is some hope of telling a story which stands on its own, and is compelling to the viewer whether he has read the books or not.
2. Ignore cutesy references to other Stephen King books / The Wizard of Oz.
Same reasons as above for the first; for the second, it ridiculously undermines the atmosphere.
3. Acknowledge you cannot tell the whole story.
Making a good (series of) movies about the Dark Tower series means acknowledging you cannot reproduce every detail and nuance of the series of seven (sometimes very long) books. Confront this fact and meditate on it. If you do not, you will fail. Begin with the idea that only a very very tiny fraction of the material will make it to the screen—in the case of a series of this magnitude, we're looking at less than 1/20th, even if, say, a trilogy is green lit. Probably less.
4. Once you have meditated on 3, pick moments from the book to collect in each of the films which (a) will stand alone, (b) are cinematic, (c) will constitute a compelling story for someone who hasn't read the books, and (d) allow for an organic and internally consistent development of the characters. How many and which moments you collect will depend crucially on the scope of the series which is funded.
For example, suppose a trilogy is funded: you might break the story down such that:
I. includes the first story in detail, the discovery of the boy, and the Gunslinger's sacrifice of the boy.
II. The Drawing of the Three? Introduce the coherence of the quest, and the secondary characters? Culminate, perhaps, in the grand train journey across the radiated countryside?
III. Some kind of climax - combining features of Wolves of the Calla, and Song of Susannah with an actual confrontation with the Man in Black at the actual Dark Tower. If the cutseyness, the NYC scenes, the meta-fictional world crossing are left out, and one focusses entirely on a) scenes crucial for character development, and b) cinematic imagery which also advances story, one may be able to compress key elements together into a coherent conclusion to the story.
Then, keep the flashback story in Wizard and Glass in reserve, to use as a prequel if one hits financial gold with the trilogy.
Key point: Don't bring "the boy" (Jake) back!!!!!! Bringing back Jake was the beginning of the end for The Dark Tower—it is the abandoning of consequences, the undermining of Roland (the Gunslinger)'s character, and the introduction of multiverse, metafictional, self-indulgent, character-loving idiocy.
Follow these rules, and a powerful adaptation could be made. Abandon them in favor of the full details of the story, and you court pathetic, self-referential claptrap. Tell a good story! This is more important than faithfulness to the exact text of King's novels. Remember that it only takes one man to create a novel, but an entire team of individuals and millions of dollars to create a movie. There is much less room for ego and self-indulgence in the realm of cinema. This brings us to
5. If Stephen King doesn't sign over the rights and agree to 1, 2, 3, and 4, then don't waste any of (our) time or (your) money, just forget about the project, and go reread the books instead.