I was fairly unaware that The Long Cosmos even existed, figuring that given Terry Pratchett died not long before The Long Utopia was released, Lobsang's voyage to the outer realms was the end of it.
However, It seems that Stephen Baxter had enough material to finish the series, and thus we get the voyage of Joshua Valiente one more time.
There's a lot happening in this one, (not that there isn't in all 5 volumes), Starting with Josh, essentially estranged from his children venturing off into the high meggers in his 70's. Nelson, the Anglican priest, gets a notice from some algorithm of Lobsang that he has a grandson. And everyone on all of the long earths gets an invitation to "Join us". Near as SETI projects can pinpoint it, coming from somewhere near Sagittarius. (Doesn't hurt that even the non human sapient species hear the call.)
The Next end up dragging in normal humans to help build The Thinker, a giant AI encoded in the Invitation.
Josh ends up in serious trouble with injuries and winds up being cared for by Trolls until his son Rob comes to rescue him. Followed almost as quickly by Josh and his troll friend Sancho rescuing Rob on an alternate Earth filled with hydrogen trees.
Nelson goes with Lobsang's adopted son into the simulation where Lobsang is currently hiding after meeting his grandson, who vanishes along with a Traveler that Nelson's family was living on stepped away.
Which eventually means every surviving major character from the series winds up on one of the Pi Earths that houses The Thinker. And eventually, the invitation is answered.
We never do get a more philosophical answer as to what it all means, although Sancho's thoughts on the matter are probably the truest of all of them.
I'm not sorry I read this, but I'm kind of happy it's officially over now.