Anthony Beevor has a book on the Civil War, but it is Beevor so rather detailed so....

Homage to Catalonia by George Orwell is a really informative read. Not so good for an objective history, but it reveals great insight into the complex make-up of the Republican side's numerous factions and political groups.

I would recommend two books (both are depressing rather than entertaining, but are accessible and are good reads). They are "The Spanish Civil War: reaction, revolution and revenge." http://eprints.lse.ac.uk/5169/ and "The Spanish holocaust: inquisition and extermination in twentieth-century Spain" http://eprints.lse.ac.uk/43152/ both by Paul Preston http://www.lse.ac.uk/researchAndExpertise/Experts/profile.aspx?KeyValue=p.preston%40lse.ac.uk