Catalonia was home to more than one Marxist or anarchist party prior to the Spanish civil war. See George Orwell’s book, “Homage to Catalonia”.
That's one book I'm remembering, but it's been decades since I read it.
Also "Mine Were of Trouble: A Nationalist Account of the Spanish Civil War" by Peter Kemp. He also fought in the war, on the Nationalist side. That one I read just 3 years ago.