This is the first book of the year that I have rated as a 5. I don't do so lightly. This is the second volume in the Revelation Space series. The series focuses on humans a few centuries after they begin leaving Earth’s solar system. They have broken into factions, generally based around how their group has adapted to life beyond Earth.
First there are the Demarchists, a group that functions under democratic anarchy. Each citizen has a neural implant that constantly pings them to vote on issues facing their society. Then you have the Ultras, a group of spacefarers who modify their bodies extensively. The final major group are the Conjoiners, whose society operates with mental augmentation that allows them to exist as a "group mind."
In the first novel, the readers learns the answer to the question, "where is everyone else?"
This second volume focuses on the first few humans who begin to understand that there is a major threat to the entire human race.
The plot kept me riveted throughout the book's 708 pages. But what I loved most were the characters — murderers, mass murderers, and petty criminals, all whose motives seem reasonable when read from their perspective and abominable when viewed through those they harmed.
The Science felt real. Everything was internally consistent, and I didn’t have to deal with anything outlandish, such as a warp drive. Humanity has just started to experiment with inertial dampeners, although not of the Star Trek variety. Space travel is restricted by actual physics. Ships accelerate to near-light-speed at 1g and the passengers feel the acceleration the entire time. The effects of time dilation are real and accounted for.
Basically, this series includes everything I love about the space opera genre without sacrificing high-quality science fiction.
If this style of fiction interests you, I highly recommend starting with Revelation Space (which I would rate at a ⅘) before reading this novel. You will want the backstory of how humanity arrived at its current place in the galaxy.