There's no doubt this is a marvellously clever book. In this final volume of the Second Foundation Trilogy, Brin gets to deliver the goods on various storylines that Bedford and Bear had to be content with foreshadowing in previous volumes: what happened to the aliens in Asimov's galaxy; why galactic society has been static for so long; what's been going on with Seldon's "Chaos Worlds"; the relevance of childhood brain fever; the origins of Gaia; and the real origin (or perhaps not, Brin hints) of human mentalic talent. Not content with that, Brin indulges in a sort of frenzy of fixer-uppery, apparently seeking to explain every oddity in Asimov's original canon, and attempting to make almost everything in galactic history the work of R. Daneel Olivaw's secret robot cabal. In particular, connections are made with Asimov's three Empire novels, "Pebble in the Sky", "The Stars Like Dust" and "The Currents of Space"; with the events (centuries in Seldon's future) of "Foundation's Edge" and "Foundation and Earth"; with Roger MacBride Allen's authorized "Caliban" robot trilogy; and with an obscure early Asimov short story, "Blind Alley". There is a clever hint that all will not turn out according to Olivaw's master plan, which nods to the reader's awareness that the Encyclopedia Galactica will exist a thousand years after Seldon's death (because we've been reading extracts from that Encyclopedia at the chapter heads of the whole Foundation series). There's a set-up for more conflict to follow on from "Foundation and Earth", if anyone cares to write the story, and even a little bit of infrastructure put in place so that we might have an improbable means of encountering Hari Seldon again.
All this plays out during "one final adventure" in the last year of Hari Seldon's life, between the recording of his last message to the Foundation and his death. This requires a little judicious rejuvenation therapy from a previously unsuspected technology, so that Seldon can briefly leave the wheelchair we saw him in when he recorded his messages. And that's an example of the problem I had with this novel - there's so much stuff in it. Characters are constantly popping up in order to reveal some new plot element, teach each other some new thing, steer the story in a new direction, or to be revealed as more than what they at first appeared. New tech is introduced so that we can encounter even more new stuff which turns out to explain really old stuff. At first I was smiling and nodding at Brin's tricksy revelations; later, as revelation piled on revelation, the story seemed to turn into a parody of itself, and I began to suffer from a ramping fit of the giggles that didn't stop until I finished the book. Now I'm just uneasy: Brin's effort to knit Asimov's canon together leaves the original work feeling weak and patchy, simply because of all the extra things Brin forces into the story while trying to fix it up.
So five well-earned stars for cleverness, but a star deducted for pushing things so far as to become funny, and another star deducted for what is essentially an elaborate (though I'm sure unintentional) undermining of Asimov's original efforts.