It's the 23rd Century and the majority of humans have died off because of harming the environment. Now, mankind has taken on a very different approach to surviving on the planet-they live with Nature, even if that means they won't live as long as they used to.
Clare is one of these humans, who lives in a small tribe. Although they exist like primitive man, they do have solar powered laptops and some of the satellites still work so they can communicate over vast distances. This is how the remainder of humanity (the majority killed off by a man-made supervirus) is able to communicate. Every few years it's a requirement that everyone has to go out on a quest, which means going out into the wild and being able to survive. Brad, a lab employee who has lead a more sheltered life than most, is forced to do this-and ends up with Clare as his guide.
It's a strange world because many of formerly extinct mammals have been reintroduced, from Mammoths to saber-toothed tigers. They were originally cloned for zoos a hundred and fifty-years before. However, mankind discovered that they were able to telepathically communicate with these mega fauna, which radically changed how people regarded the planet-and became the basis for their current society.
This is a very cool book with a lot of extremely interesting ideas. Off hand I'd say the closest thing I can compare it to is the depiction of the Neanderthal world in Robert J. Sawyer's NEANDERTHAL PARALLAX series. KNOCKING ON HEAVEN'S DOOR is sort of an anti-Apocalypse novel. It's also one of the best books I've read all year. Highly recommended.