Lilith's Brood (Xenogenesis, #1-3) by Octavia E. Butler

I'm not generally a fan of science fiction but this is one of my favourite books. It's a trilogy that I read in a weekend. I just couldn't put it down. Humanity nearly kills itself off in a nuclear war, the details of which are left mostly unstated. Out in the solar system a star ship of the extragalactic Oankali has been watching us for some time. They take the opportunity to save the remaining humans by sweeping them up and putting them in stasis for 250 years. Once the Earth has healed sufficiently they return the humans to their home planet. 

They want to interbreed with humans and create a hybrid race. This sounds creepy but the Oankali are collectors of genetic material which they use to enhance themselves. In return they give the humans genetic material that enhances their species, curing disease and extending lifespan for example. The thing they want from humans is cancer because of it's ability to replicate and grow so quickly. But can mankind overcome the Human Contradiction? That being high intelligence coupled with a hierarchical social system. These are the characteristics that nearly destroyed the human race.

Each of the books has a different protagonist, Lilith in the first book and her progeny in books two and three though Lilith remains an important character in all both of them.