^^And just to clarify, Steve is referring to earlier drafts of the book version of 2001. It follows the evolution of the storyline from "The Sentinel" (which I think is included) through to the final version (which isn't, because it's a separate book). And it discusses how Kubrick gave feedback and the project evolved.
There's some really interesting stuff in The Lost Worlds of 2001. It's not arbitrarily named. One thing that Clarke, Kubrick, and their advisors gradually hashed out was just how much to show of the aliens -- whether it was possible to create a credible depiction of alien life, or whether it was better to play it safe and leave it implicit rather than depict something that would be rendered obsolete by new discoveries (since back then SETI was just beginning and there was greater optimism that we might make actual contact before much longer). Some of the early novel drafts have much more elaborate sequences of the characters visiting the aliens' world and exploring its wonders. I think there are at least a couple of different versions of such a story in the book.
EDIT: Okay, I've just retrieved my copy of the book. (It was readily available on my bookshelf, but I was eating while I wrote the last part.) It does include "The Sentinel" as chapter 3, following the original opening chapter of the novel and a brief behind-the-scenes chapter about the genesis of the project. Then comes more behind-the-scenes stuff about the filmmaking process and Clarke's discussions with Kubrick. This is followed by four chapters from the original version of the novel, in which the "Dawn of Man" sequence is told in flashback midway through the book rather than at the beginning, and is told partly from the perspective of a fairly humanoid alien named Clindar who taught early hominids how to make the first tools. It's basically a prequel to "The Sentinel," because it ends with what's essentially a setup for the events of that story.
The next section of the book covers "The Birth of HAL," and features seven early-draft chapters in which the featured AI was a robot named Socrates. There's even an homage to Asimov's Three Laws of Robotics. We also meet the character of Senator Floyd, later Dr. Heywood Floyd (the chapters seem to come from different drafts), along with biophysicist/cyberneticist David Bowman, aerospace physiologist Dr. Kelvin Poole, and various other team members who were not just cryogenic corpsicles in this version. These chapters have a lot of setup and exposition for the Discovery mission and the scientific ideas underlying the story -- pretty much everything that was skipped over between one frame and the next in the film.
Next we get one nonfiction chapter discussing the evolution of Discovery and the research that went into creating a plausible spaceship for the film (oh, why, why is this so rare?). It's followed by 15 chapters of fiction depicting, to quote Clarke, "alternative versions of the accident, the nature of the Star Gate, and the mode of entry into it." The fiction portion of this section starts off basically where the last one ended, beginning with the launch of the ship and the long sleep en route, so we get a pretty continuous story (and again, it's stuff the movie just skipped clear over), aside from the fact that they're from different drafts. At this point, the ship's AI is named Athena. And Bowman's line "It's full of stars" goes clear back to this early draft. Except in this case, instead of a free-floating monolith, the Star Gate is inside "Jupiter V," the tiny moon which we now know as Amalthea. I think these chapters draw from two different drafts, but it's hard to tell from a cursory survey.
The final section is "The Worlds of the Star Gate," featuring chapters showing three different versions of what's on the other side. The first chapter features the return of the immortal Clindar, who gets to see the payoff for what he set in motion three million years before, and is met by the whole surviving crew of Discovery, who have come through with the ship. The next four chapters have only Bowman surviving to pass through the Gate, whereupon he (in his space pod) flies over an alien landscape and city, and is then probed and studied before finally making the beginnings of contact with an alien race, again depicted as somewhat humanoid (on the rationale that he might've been sent to a world whose natives were similar to him). The final version features more exotic aliens, in part, but is also closer to the final, more mysterious version of the ending, and the idea that the aliens' assistance to human evolution is about to take its second step.
