The Xenogenesis trilogy is compelling, sensual, and really, truly disturbing. Read in omnibus as Lilith's Brood, it is a cohesive work, troubling in both its insight and its predictions. 
 I have so very many thoughts and feelings about this story. I like a book that makes me think, and this certainly did that. Butler takes a pessimistic view of human nature, lampshading what the book calls "the Human Contradiction" of intelligence and hierarchical behavior as a fundamental, deterministic flaw. But if she's criticizing human nature, she's also using her alien characters to dig down into imperialism, colonialism, and slavery. 
 Humanity has destroyed itself in nuclear war (recall that this is 1980s sci-fi). But fear not! The alien Oankali have stumbled upon earth just in time to save humans from extinction... by genetically blending with them, whether they like it or not. 
 The three-sexed Oankali are "Gene Traders," on an endless spacefaring search for new life to incorporate into their own. All Oankali can 'taste' genetic and biochemical information, but it is the Ooloi (third sex) who can directly manipulate genetic material to change an existing organism or generate offspring from its mates' genetic material. The price for humanity's rescue is, in many ways, the end of humanity as a species, as the rescued survivors are expected mate and produce offspring with the Oankali, or live out their lives in sterility. (Human objections to this trade are universally met with incomprehension on the part of the Oankali) 
 It's a messy, complicated story, with themes that prevent the reader from simply deciding "Oankali good, humans bad." The humans tend to be nasty and brutish. The Oankali tend to be callous and arrogant--alien enough that negotiating on human terms is impossible. 
 As we move through the trilogy, we hear first from Lilith Iyapo, a human woman held captive by the Oankali, as she navigates between terrible options to carve out something of a life for herself in the new status quo. The second book is told by her Construct (Human/Oankali hybrid) son Akin, who struggles between his two identities. As Oankali, he 'knows' that the blending is really for the good of humanity, but as human, he struggles against the notion of biological limitations on free will. (view spoiler)[Ultimately, he becomes an advocate for allowing a small group of humans to settle on Mars without Oankali interference (much as one group of Oankali were permitted to keep traveling in case the gene-trade with humans went wrong). Both he and the Oankali are certain that the settlement is doomed to self-destruction, but Akin argues that they deserve the right to try to save themselves anyway. (hide spoiler)] Finally, the third book deals with another of Lilith's children, a Construct Ooloi who uses its abilities to renegotiate the relationship between humans and Oankali. 
 The increasing otherness of the narrators forces the reader to ask questions about values and consent, about interference vs. the right to self-determination, about nature vs. nurture, and about human nature itself. For the Oankali, biology is fate; to leave humans alone as they are would be as good as murdering them. For humans, the forced assimilation bears strong resemblance to a campaign of rape and genocide, and many respond accordingly. 
 The trilogy ends on a hopeful note, but without resolving every question. It's given me a lot to think about, and I'm pretty sure I'm only scratching the surface.