Ay, well, in recompense, I shall actually post for once. 'Twill be brief, but, ay, well.
Just recently finished reading the last of Octavia E. Butler's Xenogenesis (also called Lilith's Brood, I think) trilogy, which is, if one wants to examine the themes of humanity and its definition/flexibility/loss, an astoundingly beautiful and appropriate set of novels. They deal with a post-apocalyptic human race, saved from destruction at their own hands by an alien race called the Oankali. In return for their survival, the humans must lose their humanity, for the Oankali are a race of genetic traders and engineers, biologically programmed to interbreed with any intelligent race they encounter to create an entirely new species. Humans have no choice but to mate with Oankali or be rendered sterile -- humanity can now live on only as part of the genetic heritage of a new Oankali-human species. Adding even greater strangeness to this is the fact that Oankali are a three-gender race -- male, female, and ooloi; ooloi are the neuter core of the Oankali family group and responsible for the genetic engineering that governs the race's reproduction.
This all makes it sound like kind of a "humans struggling against alien rule/influence" scenario, but it's truly not. The Oankali are a wonderfully-drawn alien race, and the ooloi in particular are fascinating; the Oankali are benign and complex and have a deep care for and interest in humanity. My favorite recurring character in this trilogy was an Oankali ooloi -- and all of the novels seemed, at least to me, to be partially love stories and family stories. I don't really think I can recommend these highly enough. They're just quietly and astonishingly spectacular. (The individual titles of the trilogy are Dawn, Adulthood Rites, and Imago, although I think they are also sold now in an omnibus edition under the Lilith's Brood title.)