Niven is best known for Ringworld, one of the classic examples of a "Big Dumb Object" in SF. However there was another BDO in the novel that was seen only seen briefly, the Puppeteer migration. One of the signature moments of the book is when the characters first see a tiny pentagon of lights heading for them that rapidly expands into a rosette of five planets moving through space at 0.8 c. Fleet of Worlds, by Niven and Edward M. Lerner, finally goes back to take a look at the migration in detail.
The book is set not long after Niven's story "At the Core," in which Beowulf Schaeffer discovers that the core of the Milky Way exploded many thousands of years ago, unleashing a massive wave of radiation that is propagating through the galaxy, sterilizing everything in its path. It won't reach Known Space for another 20,000 years, but the Puppeteers, being genetic cowards, aren't waiting around. They unhook their Solar system from its star, and light out for the Magellenic Clouds. Moving planets is old hat for the Puppeteers. With a population of over a trillion, they've had to bring additional planets into their solar system to keep up with the demand for food. Unfortunately, a few hundred years before, one of those planetary movements had been spotted by a human colony ship en route to its new home. The Puppeteers captured the ship, and deposited the colony on one of the agricultural worlds to act as indentured servants, while hiding the circumstances of their kidnapping from them. When the Puppeteers flee for intergalactic space, they take the colony along for the ride.
It's now a few years after "At the Core," and Nessus (the Puppeteer who will later lead the Ringworld expedition) is training a human crew to act as advance scouts for the Fleet. Sane Puppeteers refuse to fly through hyperspace (Nessus is certifiably insane for a Puppeteer), so it seems a worthwhile experiment to have the humans do it. The human crew turns out to be excellent at the job, but as their first mission progresses, they stumble upon some evidence of their pre-colony history. What follows is a fun ride, especially for Niven fans, since a number of previously dangling threads are picked up. Nessus winds up back on Earth, setting in motion events that will show up later in both Ringworld and the Schaeffer short story "Borderland of Sol." There is finally some detail about Puppeteer society, which is long overdue.
Apparently there's a sequel due next year, so I'll be looking forward to that. Meanwhile, I'm still waiting for a story about the Pierin.