Set in the universe as Sheffield's previous novel Cold As Ice, Dark As Day is a hard science fiction novel set 30 years after the Great War, and interplanetary war that nearly destroyed the entire human race, and left horrifyingly dangerous weapons caches to threaten human survival.
The Solar System is finally recovering from the Great War - a war that devastated the planets and nearly wiped out the human race - and the population of the outer moons, orbiting Jupiter and Saturn, is growing.
On one of those moons, Alex Ligon, scion of a great interplanetary trading family has developed a wonderfully accurate new population model, and cannot wait until the newly reconstituted "Seine," the interlinked network of computers that spans the planets and moons and asteroids, comes back on line. But when it does, and he extends his perfect model a century into the future, it predicts the complete destruction of the human race.
On another moon, the Search for Extraterrestrial Intelligence goes on, undaunted by generations of failure. And to her amazement, Millie Wu, a young genius newly recruited to the project, has found a signal . . . a signal that is coming from outside the solar system.
And in his new retreat on a minor moon of Saturn, the cranky genius Rustam Battacharyia is still collecting weapons from the Great War. He thinks he may have stumbled on an unexpected new one...but he'll need to disarm it before it destroys the Sun.
Sheffield leads us through the book with his usual wit, impeccable science, and command of the language. It is hard to think of anybody who is writing hard sf these days substantially better than he is.