Dawn

Genetic engineering on what is left of the human race after a nuclear holocaust. The alien Oankali "recruit" an initially unwilling human subject, Lilith, to their cause -- both to eventually impregnate and to help teach and prepare other humans as to what is going to happen. They want to rescue the endangered humans and then resettle them on Earth, eventually helping them to repopulate via human-Oankali hybrids.

Very strange getting used to the idea (and limited description) of sexual relations with the grotesque aliens, but Butler's writing really brings the reader into the story. Worth continuing with next 2 books in the story.