The Spanish Civil War by Hugh Thomas is still regarded, I believe, as the standard work on the subject. I haven’t read it myself, because my interest in the subject isn’t so great as to call for a book running to over a thousand pages. But I’m familiar with other books by the same author and I can assure you that, in addition to his qualifications as a historian, he has the merit of writing very clearly and very readably.
Another book that is not to be missed, if you haven’t read it already, is George Orwell’s memoir of his fighting on the Republican side, Homage to Catalonia.
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