Paul Prestonís The Spanish Civil War was the definitive account when I was researching but Anthony Beevorís The Battle For Spain is also well worth checking out. For something lighter Helen Graham wrote The Spanish Civil War: A Very Short Introduction although it is indeed very short. Ghosts of Spain by Giles Tremlett covers the effects of the war on 20th century Spain magnificently.
I used to own a book of first hand accounts of the war but lost it a while back and canít remember who itís by, google is letting me down.
I also recommend George Orwellís Homage To Catalonia. While it is just an account of his experience fighting in one part of the war, and doesnít give any information about what was happening elsewhere, itís a very interesting read and provides a great deal of context for both Animal Farm and Nineteen Eighty Four.