The Dispossessed by Ursula K. LeGuin is the story of the twin planets - Anarres and Urras that rotate about one another.
Centuries ago, a group of revolutionaries on Urras escaped or were exiled to the barren world of Anarres, where they have been working the mines and building an equalitarian society - a society in which all property is owned collectively, and everyone shares everything and the government controls nothing.
Urras is a world with a beautiful main city where everyone has plenty. But the ruling class of this city oppresses the lower classes outside the city and keeps them in their caste. An oppressive government keeps the rich and poor in their places.
Shevek is a physicist, who was born on Anarres; but he is one of the few from that world permitted to visit Urras. Shevek is invited to an Urras university in order to complete his work - a work that the Urrans hope can allow them to produce faster-than-light interstellar travel. He is overwhelmed by the beauty of his host city, unaware of the suffering outside the walls until local rebels try to contact him.
The story is filled with ambiguities: The 2 planets orbit one another yet each considers the other its moon; It's never clear if walls are built to keep people out or in; the anarchy of Anarres seems morally superior to the caste system of Urras, but Anarres's system is far from perfect and its people suffer much hardships.
LeGuin tells the story in a non-linear fashion, alternating chapters between recounting Shevek's early life and his pilgrimage to Urras. This narrative style can be hard to follow at times, but it revealed his character and contrasted the strengths and weaknesses of each society.
I had forgotten how much I enjoyed LeGuin's writing. She weaves within her stories multiple themes about society and economics. I will be reading her work again soon - maybe a second reading of this book.