The Book of the Week is the first printing, paperback original of The Left Hand of Darkness by Ursula K. LeGuin (cover art by Leo and Diane Dillon).
I couldn't bear to have BOTWs about Margaret Atwood and Doris Lessing without mentioning Ursula LeGuin. One excuse often heard for excluding an author like Atwood from the science fiction genre is that her books are really about social issues. In fact, social science fiction has long been an important sub-genre of SF, and since the 1960's Ursula LeGuin has been its greatest practitioner.
First published in 1969, The Left Hand of Darkness is perhaps the best example of LeGuin's unique brand of social / feminist SF. (Next week's BOTW will be another interesting example of social SF from 1969.) The Left Hand of Darkness won both the Hugo and Nebula Awards for Best Novel of 1969. It is set on the planet Gethen (also called Winter, because it is perpetually cold), whose people have no gender most of the time, becoming either male or female for only a few days each month. Everyone is alternately male or female in different months and anyone can thus be a father and/or a mother. The result is a society absolutely devoid of sexism or gender barriers. The novel is told from the point of view of a visiting envoy who is a "normal" human, and begins with the immortal opening line: "I'll make my report as if I told a story, for I was taught as a child on my homeworld that Truth is a matter of the imagination."