Like everyone in my immediate family but my Dad, Mom was an inveterate bibliophile. Tons of books--and lots of them I have no interest in keeping, and no one else is likely to want them, so they're getting donated, but I did make sure I wasn't putting something I wanted to keep on the donate pile. The above-referenced book was one of the ones I saved out.
My first exposure to the story came years ago, when I was in my early teens. I read Clarke's The City and the Stars, which is one version of perhaps three which are extant. It was originally a novella, Against the Fall of Night, and he later made a novel of it.
Fast-forward a couple decades, and then he revisited it again--only this time Gregory Benford wrote another book which was appended to the first one. I tried reading it years ago and couldn't, because although it was sold as such, Benford wasn't writing a sequel or continuation of Clarke's story...and I'm here to tell you it's not very good.
No, strike that. It's good science fiction; I do not care for Benford's style but he does know how to write. The big problems with Benford's story are that he's not as good a writer as Clarke is, and the story is not set in the same universe as Clarke's.
Yes, there are plenty of characters from Clarke's story which appear in Benford's. Yes, the setting is highly similar. But it's not the same universe.
Clarke's story was full of the weight of millennia--eons--epochs. You could feel the oppressive weight of history wherever you turned; within a few pages of meeting Alvin and seeing your first glimpse of Diaspar, you know this shit is in the REALLY FAR future and you're not surprised when you learn that Diaspar has existed on a desert Earth for a billion years.
Alvin's perambulations through his world are suffused with that suffocating gravitas. Man achieved great things before he retreated to his hole in the Earth and pulled the hole in after him; Clarke presents a universe from which nearly all sentient life has fled, going somewhere else (Clarke himself says the alien races Man encountered went to another universe) leaving only their monuments, and a bunch of planets like empty houses.
When Alvin returns to Earth with what he's learned, though--at the end of Clarke's book--it's clearly about to spark a renaissance. Man's going to venture forth to the stars again, and start doing interesting and wonderful things...and that's where Clarke's story ends.
Benford's picks up a few centuries, or millennia, later. The hero of Benford's story is a young woman, Cley, who is approximately homo sapiens (she's got a bunch of augmentations which show up throughout the story, like eyes that can see infrared or ultraviolet, etc) and who is the last survivor of an attack by the Mad Mind.
The Mad Mind is a non-corporeal entity created by humans and aliens lo these many billions of years ago, and it's been imprisoned in a black hole for most of that time--well, it escaped, came to Earth, and killed everyone who had the same kind of genes as Cley.
...and here begins the story: Cley is dying, a raccoon-like creature calling iself Seeker After Patterns helps heal her, and blah blah blah etcetera. After this we're treated to a big panorama of all the life in the solar system, from a pinwheel-type space elevator that's alive and sentient (and made of wood? Really, Benford?) to a gigantic space-whale, to an even bigger space-whale that is a living interplanetary ship and which is an entire self-contained ecosystem.
The endpoint of the journey that Cley and Seeker are taking is the Jupiter sub-system (called "Jove" in this story) and that is where the final confrontation with the Mad Mind takes place. The climactic battle is virtually nonsensical; it's hard to figure out what the hell is happening, and why--and at the end, Seeker announces gravely that the Mad Mind has been "eaten by us" and is no longer a threat. After some denouement Cley says "Thank God" and Seeker says, "You're welcome" and I roll my eyes.
Benford's story says, "There's actually all this life clogging the entire solar system, and Alvin just didn't see any of it in the first half of the story." I'm pretty sure a pinwheel would be bloody obvious in the night sky of Earth; Benford implies that it's been there all along. He explicitly says that the big teeming confab of life around Jove has been there all along, too; but there are no hooks in Clarke's story for any of this.
Clarke's story is all about how the universe is empty. It's the end of time for Man, at least at the beginning of his story.
In Clarke's story, there's one brand of humans, just one. No "ur-humans" and "Supras", as in Benford's addition; although humanity has been enhanced and augmented by years of genetic manipulation and selective breeding, Clarke never describes what humans look like, which naturally leads the reader to assume they look like us. Benford makes Alvin (and others from Clarke's story) into a monster with a huge head and weird teeth.
Benford's story would stand quite nicely on its own, suitably edited. In fact it has almost no intersection with Clarke's story--you could probably edit Benford's story to be a standalone story in an afternoon, given the appropriate computer file. (At least, I bet I could.) Change the stuff about "the mad mind" into something else that's similar. Change all the names, except those Benford came up with, and write a few pages of exposition to explain where all this came from.
I feel like there's a point I'm forgetting, here, but the practical upshot of all this is that I would much rather have seen Clarke's vision of where Man went from the end of his book. Benford's vision wasn't it.