Summary: This book stands out from its series by retelling many of Niven's old stories, most of which are collected in Crashlander, from the point of view of one of Niven's former background characters, Sigmund Ausfaller. As a result, it's much more like a collection of short stories than a novel. It's a lot of fun for fans of the Crashlander stories, but if you are reading this as part of the "Worlds" series and haven't read the earlier stories, definitely go get Crashlander and read that first.
Forty years or so ago, Larry Niven began writing a set of stories and novels set in "Known Space" - a portion of the milky way that was at least partially explored by the human race. Since then, he has occasionally let other writers into his playground, particularly in the The Man-Kzin Wars series.
This time, Niven has collaborated with Edward Lerner to write three books about the "Puppeteers", a race of technologically advanced herd animals, and their interactions with humanity.
The first book, Fleet of Worlds, re-introduces the Puppeteers fleet of five (now six) travelling planets for readers not familiar with them, and takes place largely on those planets, as does the third book, Destroyer of Worlds. This is the second in the series, and in some ways, the most daring. It is closer to a collection of short stories than a novel, and follows the connections of Sigmund Ausfaller, an existing Niven character, with the puppeteers and the fleet of worlds.
Previously, Ausfaller had appeared in the background of several Niven short stories, most of which are collected in Crashlander. In those stories, he's a mysterious agent of Earth's "ARM" intelligence agency, who typically shows up to manipulate the hero of thoses stories, interstellar pilot Bey Shaffer. Juggler of Worlds re-tells those stories from Ausfaller's perspective, plus one story ("The Soft Weapon") from the perspective of Nessus, a puppeteer "scout" who is also a long-standing Niven character. If you have fond memories of the Crashlander stories (and I do), rereading them from another perspective is a neat experience, but if you haven't read them, some of these stories will be challenging. If you are picking this up because you are reading the "Worlds" series but haven't read Crashlander, definitely read that first. You would also be well advised to read The Soft Weapon, but that's much harder to find.
Overall, the stories worked for me. I liked the growing relationship (often behind multiple levels of deception and proxies) between the two agents, Nessus and Ausfaller, as well as their parallels. Nessus, from a race of obsessively cautious herd animals, is made a scout because he is insanely daring (for a puppeteer), and Ausfaller, from a race of curious primates, is made an intelligence agent because he is insanely paranoid (for a human). The book does a nice job of sketching out their loneliness and their ongoing intelligence duel.
I have a few gripes that prevent me from giving it five stars. First, I was a little disappointed that the "lone genius" trope applies to puppeteers too. We are told several times that the puppeteer home world has TRILLIONS of puppeteers, but we meet fewer than ten: Hindmost (the leader of the civilization); Achilles (another scout); Baedeker (a genius engineer); Nessus; and a few supporting characters.
Baedeker, in particular, is the Wesley Crusher of the puppeteer civilization. If you need a previously nonnegotiable law of physics repealed, you apparently just call Baedeker. Presumably, with a civilization of a trillion plus hyperintelligent herd animals, some kind of distributed problem solving would be the way to solve problems - imagine what you could accomplish with a well managed set of one hundred teams of a thousand scientists each, plus logistical support. Ten thousand teams? A couple lines about Baedeker using the herd would have been great.
Still, I'm very glad I read Juggler of Worlds, and really enjoyed it, particularly the chance to revisit some of my favorite stories. Read it, but read Crashlander first.