I first read Thomas back in 1966 or 1967 when doing so could get you in a lot of trouble. I’ve read it a few other times over the years and it holds up pretty well. Another one I really like is Ronald Fraser’s Oral History Of the Spanish Civil War. Terrific book. Another one my father gave to me and he was a big SCW fan (he went to a lot of battlefield sites in and around Madrid when we were living there but I was in college) is Dark and Bloody Ground: A Guerilla Diary of the Spanish Civil War by F. Perez Lopez. It reads more like a fictional, picaresque history and we could never decide if it’s fiction or history orca combination of the two. Either way, a great book. You can still find some copies on eBay.