It's been a month since The Dark Tower was released in theaters. I watched it opening weekend, and I must say, I don't think it was all that bad. Yes, there were some mistakes, but when it comes to films, nothing's perfect. There are always going to be plot holes and various things that can't be in the final product, which is to be expected, especially when one is familiar with the material, which is how this movie approached the subject. Anyone who hasn't read the books is going to be lost and a bit confused by the events, due to the short run time. It's rather difficult to fit everything into an hour-and-a-half.
I've read the Dark Tower books from the beginning, but it's been years since I finished book seven. I'm not an expert in the field of memorizing everything that happened in the pages, but there were definitely things missing. Someone had mentioned in a comments section of the movie review on another site that this could be taken as a new iteration from the books. Which makes sense, if you think about it. If you don't wish to read any spoilers, I'd suggest you just skip to the last paragraph, because I'm going to go slightly in depth with what I've seen from the film that is different from the books.
First off, it seems they tried to condense the entirety of all seven books into one film. Roland was able to catch up with Walter in The Gunslinger, where he has a conversation about future events in a Tarot reading. Jake makes it to Mid-World through the abandoned house, but after joining with Roland, falls from a cliff, due to a Beam quake. However, Roland catches him, but is unable to pull him to safety, resulting in his "death." Jake also tells Roland to let him go, because "there are other worlds than these."
This is what causes Jake to dream of the Dark Tower and Roland. They don't happen out of nowhere.
The next book is when Jake is able to return, along with Eddie from New York, and Odessa/Susannah, a legless woman with a dual personality. Also, during their travels, the group meet up with a creature known as a Billy-bumbler, which has limited speaking abilities, only able to say, "Oy," which becomes its name, and "ake" trying to say Jake's name. The last three characters were omitted from the film, as there was a lot going on with them.
This is also where Roland loses two of his fingers on his right hand to a creature of the sea, causing him to be poisoned from the bite.
The people with the false faces don't appear until books five through seven, where Jake learns of their attempts to destroy the Tower. In book six, Song of Susannah, Stephen King himself makes an appearance, as the group needs to meet the writer in order to understand how to reach the Tower, for Roland to fulfill his quest.
Finally, in book seven, The Dark Tower, Roland is able to reach his destination, calling out all who traveled with him to reach this part of his journey. He climbs the stairs and approaches the door at the top of the Tower. Once he enters the door, it goes all the way back to the first line written in The Gunslinger.
This is where the film picks up, so it would seem that this is a new part to his quest. So, if you're debating as to whether it's worth buying, wait until it comes out on DVD/Netflix/Amazon and rent it first. There are good moments in the film worth watching, especially if you like looking for the different characters spread throughout to see how the universe is connected.
"The man in black fled across the desert, and the gunslinger followed."
— Stephen King, The Gunslinger