I wouldn't say that it is explicitly Anti-Capitalist/Pro-Socialist, but the Wiedźmin (Witcher for the anglos) book series has some pretty cool anti-imperialist vibes in it. It is actually extremely different in politics to the games by CDPR and is generally better than most Euro fantasy in its treatment of racism and nationalism. I think the author has some anti-soviet vibes, which didn't matter that much to me at the time of reading, but I guess are worthy of note for many here. Books are on libgen in many languages, and the audiobook versions by Peter Kenny are very well acted.
I also haven't gotten around to finishing Chernyshevsky's "What is to be done" (not the Lenin one) but I liked it so far. It has also been a while, but Ramos's Vidas Secas was one of my favourite books as a child, but only recently I learned he was communist. Not sure how that last one translates to English though.
Also since you mentioned Animal Farm, I've head that "Brave New World" is generally regarded as an "opposite day 1984". Can't attest for its quality though, as I haven't read it yet.
Also Neal Stephenson has weird thoughts and I wouldn't consider him a socialist, and his books kinda read like having a bad trip while reading historical conspiracy theories, but Diamond Age kinda has a "broken clock correct twice" aspect to it. Kinda like the "Cocaine-phase Stephen King" of sci-fi.