I have read Combat Over Spain by Jose de Larios. He presents more truth about the origins of the war in about 12 pages than in any twelve volumes at a typical public library.
Homage to Catalonia by George Orwell is essential reading for the period too. Very fascinating, as Orwell was a dedicated, Trotskyite leftist who barely escaped with his life from his more Stalinist “comrades”.
The Passionate War by Peter Wyden is left of center, but not insanely so, and does candidly present a lot of the chicanery flourishing between the Kremlin and Madrid, and puts in proper context the relative amounts of ‘foreign assistance’ from Moscow and Paris on one hand, and Rome and Berlin on the other. It was interesting to read of the transfer of virtually all of Spain’s gold bullion from the treasury into Soviet vessels. Quite Wagnerian. Ironic that some of Moctezuma’s gold may now be in the hands of Soros.