Don't get me wrong: I like Clarke's book (and several of his others). But:
There are things in the book not in the movie. The book will not help you with them.
There are things in the movie not in the book. The book will not help you with them.
It isn't just that the two works exist in different media, that one has music, sound effects, etc. and the other doesn't.
The book and movie frequently go their separate ways. The book and movie, even when they do contain some of the same "stuff", contradict each other on big and small things. The book will not help you with any of that. They contradict each other on big and small plot points but also in many other ways having nothing to do with the plot but in some things potentially more important than the plot.
If you watch the movie only for its plot, you may well think parts of it are monotonous or boring. You would be wondering why it does not offer you what you want even as you ignore what it does actually offer you. It isn't like other movies. It is most certainly not its plot. Other posters here assert e.g. "the weird kaleidoscope crap", etc., is boring. This is evidence they want only plot, appreciate only plot. I have news for them: the "weird kaleidoscope crap" is no different from anything else in the movie. The entire movie is weird. It is very easy to document. They are not aware of what they are experiencing. And thus they are not aware of meaning. They are not enjoying it because they do not see what is right in front of them. The supposedly boring "weird kaleidoscope crap" is not an artsy-fartsy affectation the film maker put into the movie in an attempt to convince viewers it isn't as boring as it seems. It is fully consistent in many ways with scenes before it and after it. The movie does not suddenly get weird at the star ride. Anyone who asserts that isn't paying attention. It may be the strangest Hollywood movie ever made. All of it.
What you may end up thinking after reading the book is that Kubrick attempted and failed to put Clarke's ideas on screen, that Clarke's book will save you from Kubrick's incompetence. I realize those who recommend the book never phrase it like that but that is a very possible consequence of it. You may end up thinking that it is to your benefit to see Kubrick's movie through Clarke's eyes. I think that is a big mistake.
On the other hand if you watch the movie carefully and many times and read the book carefully and many times--no, I don't expect you to do either--you may realize something very interesting about their relationship. I doubt it is something someone who suggests "Read the book" realizes. (I strongly doubt anyone who says "Read the book" has seen the movie many times and read the book many times.) I suspect such people are not very familiar with either work; I suspect that they think the book is a handy crutch, a Cliff's Notes for the movie.
I give the same tidbit I always give:
"[Bowman and Poole] could enjoy what tasted like--and equally important, looked like--orange juice, eggs (any style), steaks, chops, roasts, fresh vegetables, assorted fruits, ice cream, and even freshly baked bread" (Clarke p.100, Clarke's italics!).
Is that what we see on Discovery when Bowman and Poole eat? Those who recommend "Read the book" have no answer here. They can only ignore the question. The book will not help you here.
Of course, a viewer might think "Help me with what? It's just a few boring shots of a couple of guys eating futuristic food, something simple and obvious Clarke handles in a sentence or two. The movie is a filming of the future and this is an example of it. The plot picks up the pace once the interesting part of the plot with HAL starts." If a viewer thinks that way, he is not appreciating what is going on in the movie and I do not refer to its plot. And this is one of hundreds of such instances.
If the plot of Clarke's book explains the plot of Kubrick's movie, then Kubrick failed badly here to put those words into images. Note that expensive and difficult to create special effects are not the issue in this instance, but only a couple of steaks and some ice cream. For those who follow only the plot in a "Read the book" mode, this must be evidence Kubrick is incompetent. Right?
To that I might add as a second example: on what page does Clarke's novel explain the creature throwing the bone up into the air? Indeed, on what page in the published screenplay is that scene? A viewer might assert there is nothing to explain about it--that's why Clarke doesn't discuss it, that's why Clarke has his character use a stone and not bone, that's why Clarke has many Earth monoliths instead of just one, that's why etc. etc. I can demonstrate that assertion is not helpful if one wishes to enjoy the movie.
Well, what else might there be in a work beside a plot?
If the symbolism of Clarke's book explains the symbolism of Kubrick's movie ..., oh, Clarke's book has virtually no symbolism. What little symbolism it has he dutifully explains. He explains everything. He even comes close to attempting to explain to the reader that he cannot explain what he cannot explain at the book's end. (He discusses more of that in Lost Worlds of 2001.) Clarke's book will not help you enjoy the movie but may end up either convincing you the movie is even worse than you thought it was (e.g. that even the apparently non-monotonous, non-boring parts really are boring, etc.) or that the book does indeed explain it when that cannot possibly be true. You lose either way.
In the OP you write the movie has monotonous stretches. "Drawn out" you write in another comment. I have seen the movie many times and not a second of it bores me. The meaning of each shot echoes or mirrors other shots. The visual or audio content echoes too. And yes, each part of the plot echoes or mirrors other parts of the plot. But if you pay attention only to the plot, waiting for something interesting to happen, you are missing wonders right in front of your nose, things even more astonishing than the stuff in the plot. It isn't like other movies. It has almost nothing in common with e.g. Interstellar, Gravity, or Star Wars.
It may be the most complex movie ever made. No, I do not refer to the technology the film makers used to create it. I refer to what it may mean. In Star Wars a spacecraft is just a spacecraft. It may have taken millions of dollars and weeks of computer time to generate it but it is only one thing, it means one and only one thing. Even right after the premiere of 2001 many viewers realized that is not true of 2001. You don't know what I mean, do you?