Left Hand of Darkness by Ursula K. Le Guin
What would it be like on a world where everyone was an asexual hermaphrodite 99% of the time? Where gender and sex were not the primary focus of day to day life, because most of the time everyone was the same in this regard? Left Hand of Darkness by Ursula K. Le Guin thrusts Genly Ai, a run of the mill male Terran human, into the middle of the planet Winter where the inhabitants fit this description. His mission is to convince them to join the rest of humanity in a share of information. This book really plays with the idea of what it means to be human, and the role of gender and sex within that. It tries to tease out those things which may be constant, and those things which are due to a duality which has been baked into our culture. Le Guin also does a great job at world building without it feeling false, and just tying all of the characters together. It is an excellent book with an excellent moral.