I'd hesitate to call "The Dispossessed" a science fiction book. Of course, on its face, it appears to have many of the trappings of the genre: alien planets; strange futuristic technologies; a certain kitsch. But its subtitle: "An Ambiguous Utopia" perhaps hints at its transcendence of the genre.
Le Guin's novel is built on juxtaposition, on contrasts. It's set on twin planets Urras and Anarres. Moons of each other, they resemble Yin and Yang. Urras is lush, green, and brimming with life. Anarres is a cold, desolate, inhospitable desert planet. The book follows brilliant Anarresti physicist Shevek who travels to A-Io on Urras with the stated purpose of furthering his research - a novel theory that unifies time ("The Principles of Simultaneity") and that acts as a sort of MacGuffin. Although quite naive, Shevek eventually comes to realise that he is playing into the machinations of the Urrasti political class.
In reality, the plot is secondary. Le Guin's book is about ideology. On Urras, A-Io has a patriarchal capitalist system. Thu (also on Urras, though only mentioned in passing) is an authoritarian communist state (with whom A-Io is at war). Annares, on the other hand, is an anarcho-syndicalist society, with an emphasis on free association, mutual aid, and cybernetics.
The great genius of this book is in Le Guin's handling of these systems, particularly the anarchist Anarres. Anarchism is a political philosophy that (at its most basic level) rejects hierarchy, emphasises cooperation and voluntarism, and is focused on preventing power from centralising. Underlying it is an alternative theory of Hobbes' 'State of Nature' which, rather than asserting that humans naturally aim to maximise their chances of survival (often violently) and thus require a state to keep them in check, instead asserts that human nature is to cooperate and compromise. Le Guin fleshes out the anarchist world with a deontological moral framework and a culture based on Anarresti ideals, including their (sometimes alien) attitudes to sex, child-rearing, work, punishment, trade, education, and ownership.
Anarchists are often seen as idealistic (and with good reason). Whether you sympathise with their cause hinges on how you view human nature: are we inherently altruistic, or inherently selfish? Le Guin's anarchist society is not without its problems: while official hierarchy does not exist, there are still shadow power structures (notably with Sabul, who controls what is published). Ideological purity is held as a virtue, and while there is no 'punishment,' social ostracisation is a very real possibility (and one experienced by Shevek due to his decision to leave Anarres). Work is carried out only through social obligation - a kind of utilitarian ethics that places 'the greater good' over personal wants. For Le Guin's society (and, by extension, the broader anarchist project) these issues aren't intractable, but they require assessment and cultural reorientation. Anarres as the titular 'Ambiguous Utopia' is a society with absolute freedom in principle, but with powerful social norms and few resources, functionally limiting these freedoms.
On Urras, Shevek's interactions with the capitalist A-Io highlight the inadequacies of capitalism in sharp relief. The effect of social class permeates everything in A-Io society. Shevek's diatribes on the moral bankruptcy of profiteering fall on deaf ears - the people he is given access to are those for whom the system is set up to benefit. It's not until he escapes the gilded cage of the university that Shevek can interact with the proles; the downtrodden worker class who, out of sight, labour to allow the privileged to lead the life they lead. For the privileged, perhaps A-Io is the titular 'Ambiguous Utopia,' but can we call a system utopian if it relies on ignorance?
Under capitalism, we are told we are free. Our own ruling classes deploy armies of economists to convince us that the free market is man's greatest invention; the most efficient way ever devised to allocate resources where they are needed. The meritocratic myth of social mobility is repeated ad nauseam. In reality, we are prisoners of circumstance. The free market is an engine of inequality. Capital pools at the top and the apparatus of the state quickly stamp out 'dangerous' ideas that threaten the integrity of the system. The incentive structure of capitalism drives this - there is no conspiracy; no shadowy cabal of profiteers at the helm. There is just one singular motivation: wealth accumulation. These facts dawn on Shevek throughout his time on Urras. His contribution to the marketplace of ideas ends in state violence against thousands of protestors. He realises lofty ideals such as 'the greater good' or 'scientific progress' have no bearing on capitalists unless they are in service of capital. And he at last realises that his gift to the universe - the unified theory of time - will be used for domination if it is solely in A-Io hands.
Although published in the 70s in the wake of the Vietnam War (the ongoing proxy war in the book between capitalist A-Io and communist Thu is an obvious allusion), the themes in "The Dispossessed" have only gained relevance as capitalism's global hegemony has grown and metastasised. If, as Mark Fisher claims, we live in an age of capitalist realism, then our collective imagination is the only way through. Le Guin's imagined future (and others like it) could be a catalyst, or a warning.