Imago is the sequel to "Dawn" and "Adulthood Rites", though in no way concluding. It tells the story of another of Liliths children, Jodahs, set roughly 50 years after "Adulthood Rites". Jodahs is truly alien, and yet its humanity makes it alien to the aliens, too. While Jodahs (and its companions)struggles to find its own identity, which is not exactly hybrid, as in, part alien and part human, but something new and beyond the duality of human/alien, Octavia Butler lets us, the readers, struggle with the concept of a gender that is beyond the duality of male/female. What raises "Imago" above most other stories of third genders is that by the end of the book it feels like the way it should be, most natural, nothing exotic about it anymore. It becomes human. Or rather, it is beyond classification as human or alien.
Way further developed than any ideas of cyborgs or other hybrids I've read so far, "Imago" doesn't stand between boundaries or break them up, because in "Imago" the boundaries cease to exist.
This superbly worked out philosophical background helps over the dissappointment of very little reference to Lilith, Tino, Nikanj, Akin, and other characters of the first two books. If you've read and enjoyed them, read "Imago" too. It's part of "Lilith's Brood", if you can't get it as a single book.