The Left Hand Of Darkness by Ursula K. Le Guin (1969)
Genry Ai is an alien envoy to the icy planet Winter. In the midst of political conflict and cultural differences, he attempts to recruit the planet to join the Ekumen, a confederacy of spacefaring planets.
The largest difference between the humans on Winter and those of other planets is that for most of the time the humans on Winter are genderless, and only become gendered during a monthly phase of sexual potential called Kemmer. During these phases they will choose a gender depending on preference or the circumstances and one person may be male during one month's kemmer phase and female during the next.
I loved this book. Really good. I feel like I just want to read everything Ursula K Le Guin wrote.