Kubrick was fairly known for heavily adapting the novels he used for inspiration. There are huge differences between the novel "Lolita" and Kubrick's adaptation. Same with "Shining" or "Clockwork Orange".
TBH, I'm really happy we didn't get a version with voice-overs and all. Hard to imagine a concept of "documentary" would have been better than what we ended up with.
Stephen King is publicly critical of Kubrick's treatment of "The Shining", mostly about its unchanging and one-dimensional characters. While I expect "2001" would not have been improved by a lot of explanatory voiceovers, I think "The Shining" would be seriously improved by Jack not being obviously crazy right from the first.
Most authors complained about it yes. Burgess accused Kubrick of not having understood a single word of "Clockwork Orange". Nabokov was the most gracious when he said that while Lolita was a very good movie, he didn't recognize his original work at all :)
I think it's a very difficult exercise to adapt a novel. Should you literally put on the screen what is written in the book ? What's the point ? You're not creating much. On the other hand, if you deviate from the original work, fans start attacking you for "betraying" the spirit of the book.
Billy Wilder told the story of adapting Stalag 17. Co-writer Blum showed up the first day with a copy of the original play and said “I thought we could use this.” To which Wilder showed him how they could use it: by dropping it on the floor and using it as a door stop.
Don't forget Dr. Strangelove, which is based on a very serious novel about nuclear war, "Two Hours To Doom" (published in the US as "Red Alert"), which Kubrick and Terry Southern turned into a comedy. (Today the book is out of print and largely forgotten except for its relationship to the film.)
The Godfather was pretty close to the novel, with much dialog line-for-line identical. The main difference was the subtraction of a few egregiously daft subplots.
Hum, there was also a lot of differences with the importance of the character of Johnny Fontane who is way more present in the novel.
In this instance, the "opposite" actually happened. Coppola is supposed to have said, when finishing the last page of the novel, "I'm supposed to make a masterpiece of this turd ?"
Steven King got to write the screenplay for his own version of the shining in the form of a TV miniseries. It was so bad it was actually funny instead of being scary. I think it is safe to say here that history has proven Kubrick right on this particular point.
Eh. Unlike the previous commenter, I didn't like the miniseries "way more," but in certain respects it's an unfair comparison--a workmanlike director rather than Stanley Fucking Kubrick, an overlong script treated as more or less untouchable, and what was, when push comes to shove, a TV movie budget for special effects. This affected nearly all of King's TV adaptations, with the possible exception of "Salem's Lot," which had the good fortune to be directed by Tobe Hoober. (For all the fondness the original "It" is remembered with, it's...more unintentionally comical than I think people remember, too.) Even so, I would argue that the miniseries does a better job of capturing the novel's themes about alcoholism and mental illness than the movie does. If they'd had a slightly more inventive director--and both the willingness and contractual freedom to make it a two-part miniseries instead of a three-part one--it would have been pretty great. Not Kubrick great, no, but great.
(Also: Steven Weber is a better Jack Torrance than Jack Nicholson is. Fight me.)
My understanding is that 2001 isn't so much an adaptation of the novel as it is a script that was written through extensive collaboration between both Kubrick and Clarke, eventually adapted into both screenplay and novel. I've got quite an interesting book that goes into some detail about it - "2001: filming the future"
I think it may have been in there that I read something to the effect of "Kubrick withheld his approval of the book because he didn't want to hurt the release of the film."
TBH, I'm really happy we didn't get a version with voice-overs and all. Hard to imagine a concept of "documentary" would have been better than what we ended up with.