This is like saying "Batman Returns" has taught me all I need to know about the practices of US law enforcement. (It seems that US law enforcement is largely carried out by independently wealthy martial arts hobbyists with latex fetishes.)
Das Kapital is not a book about Marxism, and it is not fictional, though it is wrong about some things. Marxism is a movement about Das Kapital; consequently you can learn a great deal about Marxism by reading it.
The Dispossessed is a work of fiction; it tells you what was in the mind of its author. But, like Das Kapital, it has built up a certain following, and you can probably learn a lot about that following from reading the book, just as you can learn about cosplayers by reading (or watching) Batman. However, this doesn't tell you much of anything about the possible ways human society can be organized.
AFAIR "The Dispossed" assumed people will accept radically lower standard of living, if the system itself is "fair" (low inequality). Communism implemented that in pratice (low inequality, but also low standard of living) and people hated it.
None of the regimes that used the label "communist" in our actual history have ever implemented anything like that in practice. In USSR under Stalin, families of well-off party members (not just political elites, but valuable specialists as well) had domestic servants - not even in a wink wink nudge nudge kind of way, but all completely legal and official.
Communists, and statist socialists in general, are actually represented in the book as the state of Thu. And it's implied to be just as nasty and authoritarian as the real deal.
Polish communist apparatchiks were better off than regular Poles, but they essentially lived the life of middle class American (a house, car, maybe summer cottage, yearly vacations in Bulgaria or some other place with a sea and a lot of sun). The inequality levels were low back then. Nevertheless, people hated it.
The important thing in this case isn't the absolute degree of income inequality, but rather how it is perceived. When state propaganda talks non-stop about how everybody's equal in this new paradise, yet every day you see things - even little ones - demonstrating that some animals are manifestly more equal than others, that can grate a lot more than a similar relative difference in a society that openly puts inequality on its shield.
And there was no shortage of those little things. Well, I can't speak for Poland, but even in the late USSR (which was comparatively less unequal than Stalin's one, ironically), there was still blat and Beryozkas.
