I’ve been encouraging people here for years to read Orwell’s account of his time in that milieu, Homage to Catalonia. It is a quick read and is a great introduction to and first hand account of the Spanish Civil War. Orwell barely escaped with his life, not due to the Nationalists, but at the hands of his Communist allies in the purge you mentioned (he was an ideological socialist fighting with the anarchists). You can see this experience leading directly to his vastly more famous Animal Farm and 1984.
Yet our government insisted on maintaining the myth that the “Loyalists” were the good guys. Movies like “Sahara” and “The Guns of Navarrone” portray the Franquistas as Nazis...