Aside from 'Homage to Catalonia' by Orwell, the only books about the Civil War I've read are the Falcó novels by Arturo Perez Reverte. They are a pulp noir series about a James Bond-esque character that works for the Franco side. There's more sex than historical context, and the author constantly reminds us that the protagonist is a mercenary that doesn't really care about politics, but it is still a unique point of view.
By the way, this author has published other books about the Civil War, both fiction and non-fiction, but I haven't read them. You can find them in the "related" recommendations in this link to the first e-book in the Falcó saga:
https://www.amazon.es/Falc%C3%B3-Serie-Arturo-P%C3%A9rez-Reverte-ebook/dp/B01LVXNX1F/ref=sr_1_6