This one is the pay-off on four previous "Worlds" novels, co-written by Niven and Lerner, as well as continuing the "Ringworld" story Niven has told in four novels of his own.
That's a lot of background, and the authors quite rightly don't rehearse it all for new readers. As someone who has read all eight precursor novels, I spent much of the first third of the book wracking memory for the necessary background as previously known characters and situations slotted into the new story. When time permits, I'll perhaps sit down and read all nine in order - there's a sense of missing the occasional reference in this story when working from old memory.
The latter two-thirds of the novel builds to a complex and involving climax, although the general shape of the ending is evident to the reader for some time before the pay-off.
The authors leave one thread dangling suspiciously, and leave major characters scattered in five separate locations, any of which could potentially take the story forward. Although Niven foresaw the Known Space universe finally sinking into the stagnation of his short story "Safe At Any Speed", there's still a couple of centuries in the Known Space timeline before that happens. The authors still have plenty of room for more developments in the current storyline, if they choose to exploit it. I hope they do.