Reading the prefatory material to this book, it sounds as if Clarke actually wrote the screenplay to the film with the director Stanley Kubrick before he wrote the book. So in a sense, this book is an adaptation or an extended treatment. And it's impossible not to compare the two works. One is a classic movie, engaging in parts and a mind trip in others. Clarke's book lays out the principles of the movie in my more dogmatic and thus less-interesting form. That's not to say that the book isn't intriguing in its own right.
Take, for example, the beginning sections of the film and the book, which deal with prehistoric man. I find those sections of the film rather dull. For one, one doesn't quite understand what's going on among these prelinguistic apes, and such doesn't make for the most-compelling viewing. In the book, the thoughts of the lead ape are described in detail, and you get the sense of discovery that early pre-man has hit upon: hunting, not just gathering; tools; killing others.
Skip ahead to the future, and man is out exploring space. When a strange monolith is discovered on the moon, similar to the one that the apes discovered, the United States decides to send men to Saturn to study the monolith's possible origins. Here, the film and book fairly well match each other, though the book goes into more detail. Clarke is masterful in predicting future technological advances, though some things seem strangely anachronistic as well (a video phone hooked up by cables?). Both the film and movie do a good job of explaining how technically some things might be accomplished, given enough manpower and will.
This middle section if the most-compelling part of both works, as a computer that runs the spaceship that is taking five astronauts to one of Saturn's moons decides that its human counterparts aren't up to the job--and that their presence threatens its own existence. Herein, Dave Bowman does all he can to save his crew, himself, and his mission, as the plot centers on man versus self-cognizant machine.
It's the last section that I was most curious to see "adapted." In the film, this becomes a masterful 1970s tripout. What in the world could Clarke have done in the book? Well, it becomes a description of the universe and of the possible origins of man--a lecture of sorts, of far less interest. And of the ultimate ending? In the movie, with less explanation, the scenes of Bowman hanging out in a room are mysterious and strange. In the book, all of this is explained. Strange, sure, but not as mysterious. In all the tripout is replaced with something much more in keeping with science fiction and with the thoughts of someone whose done a lot of thinking about the subject of intelligence, which makes for much less ambiguity and thus for something that is not quite as much of a classic.