The starting point for the story is the regularly reprinted short story, "The Sentinel", included within. It basically represents the portion of the story of the monolith on the moon, a marker left by an advanced alien race that would be triggered only if man made the one small leap into his larger universe.
Of particular interest to me were a few chapters of an earlier draft of the book, focusing on the Dawn of Man sequence. At one point, the plan was to actually have an alien visitor interacting with the ape-men. Despite having access to a fine make-up artist in Stuart Freeborn, I do think it was a wise decision NOT to pursue this path in the film, as I can't imagine any sort of alien design that would not have taken the viewer out of the moment. That said, it is very interesting to read these chapters, told from the alien (Clindar's) perspective.
Additional unseen chapters detail the life of the astronauts on Earth before heading to Jupiter, and a number of alternate events that befell a larger group of astronauts in the original concept. The final chapters of the book provide an alternate series of events that occurred once Bowman was through the Star Gate. The first of these reintroduce us to Clindar. The sequences, which attempt to describe both the aliens and their civilization – would definitely not have worked onscreen, and they don’t hold up very well in the novel format, either.
All in all, the book offers an interesting insight into the development of the film and novel. It’s a rare opportunity to be able to read alternate chapters of a book. If you’re a fan of the film (and novel), The Lost Worlds of 2001 is definitely worth tracking down.