This is a first-hand account, is easily readable and through a quirk of circumstances he gets to see a lot of the story.
The best account I ever read of the Spanish Civil War was Burnett Bolloten's 1107 page "The Spanish Civil War". Bolloten was an American UPI correspondent in Spain in 1936 and felt that the truth about what went on there did not make it out during those years, so he made it his lifes work to tell the story and in 1987 (51 years after the outbreak of the war), he published his book. It covers everything - the rural communes, the politics between Russia, England, Spain, the US and France...pretty much everything.
-- Lance