Ursula K. Le Guin, The Left Hand of Darkness (1969, re-reading). Since reading The Inland Whale by Le Guin's mother Theodora Kroeber, a retelling of nine California Indian legends, I came to perceive much of Le Guin's science fiction as fan fiction based on her mother's book: she describes humanoids slightly biologically different from Earth humans, and imagines their social organization, religion, legends. In this novel, the humanoids are neither inherently male nor female: they are sexless three weeks of the month, and can be either male or female during the remaining week (improbably, the year, day and month on their single-moon home planet are almost of the same length as on Earth); a pregnant or lactating humanoid stays female, but after the child is weaned, goes back to sexlessness. The genetics of it are never explained, nor is the lack of prohibition of incest except when conceiving children. It is hinted that the native humanoid protagonist broke the taboo when he/she had a child with his/her brother/sister, which caused the brother/sister to commit suicide; when the Earth human protagonist teaches him/her telepathy, the voice in his/her head is the dead brother/sister's.
Other than this concept, this novel is mostly an Arctic adventure story and a boring tale of political intrigue involving two governments, a medieval one (a sentence having to do with it made it to Bartlett's Familiar Quotations: "The king was pregnant") and a cartoonishly totalitarian one. The writing is beautiful, though.