The Left Hand of Darkness, Ursula K. Le Guin
Somehow I never managed to pick up this classic story until now.
Genly Ai is the envoy of the Ekumen (an interstellar affiliation of planets) to the planet Gethen. What makes Gethen unique (aside from its ice-age climate) is that its people are ambisexual, not divided into two sexes. Gethenians are basically asexual (or only latently sexual) for most of each month, until they enter kemmer. In kemmer, a Gethenian might take either the male or female role in sexual relations and childbearing.
Ai is trying to get either of the two main powers on the main continent to agree to join the Ekumen. Karhide might be described as feudal, but Le Guin carefully shows how non-sexual psychology modifies feudal political relationships. The description of Orgoreyn, on the other hand, reads like a gloss of The Gulag Archipelago, although in fact this book predated the publication of Solzhenitsyn’s by four years, and again there are differences from the Stalinist model due to the Gethenian psychology.
If the heart of SF is literature that takes a technological premise and explores its ramifications, The Left Hand of Darkness might be considered the quitessential SF novel. Even in the final chapters, new facets of the Gethenian psychology and society, derived from their unique biology, are still being revealed. This is not just a chapter or two of scientific or technical premise tacked on to the front of a long action story, but a speculative exploration from front to back, and that is what really made the book shine for me.