Hugh Thomas' The Spanish Civil War: Vols I & II are a good starting point, though perhaps somewhat dated. /u/cordis_melum would be good to consult.

As the others have said, Hugh Thomas isn't a bad starting point. He keeps updating his work, so even though it's a bit dated it has incorporated a ton since its original release in 1961. Antony Beevor may be a bit more accessible and a touch less biased. Paul Preston's work is more biased (in favor of the Republicans) but is outstanding on the ideological aspects of the war. That being said, skip The Spanish Holocaust until you understand the broad brushstrokes of the war. Stanley Payne also has a good one-volume history.

There is also the excellent The Spanish Civil War as a Religious Tragedy by Jose M. Sanchez. I always mention this book because it is the English-language book on the anticlerical violence of the war (While Preston's Holocaust covers all of the other violence against noncombatants). Unlike works that are apologists for Franco, Sanchez provides a balanced view of the religious aspects of the war. He examines the underlying causes of the violence against clergy, the ideological issues at play that led many Spanish clerics to support Franco's regime, the implications of this support (before, during, and after the war), and those clergy that did not support the Nationalists.

Where you need to exercise caution is with just about anything written in the immediate aftermath of the war. Orwell and Hemingway are great, but they were directly involved and sometimes can't see the forest for the trees. That's dangerous because their experiences shouldn't be extrapolated for the entirety of the war, but it's incredible at the same time for giving us a very literate first-hand experience.

Anything with "Crusade" in the title or on the back cover should be ignored. Sanchez dismantles that argument in his work.

As already said, Hugh Thomas' The Spanish Civil War is almost a bible of everything to do with the war and certainly an excellent resource on the war as a whole. However, for a better easing into the topic, I'd recommend Antony Beevor's The Battle for Spain and, especially good for the more social and political side of things, Paul Preston's The Spanish Civil War: Reaction, Revolution and Revenge.

There're thousands and thousands of books focussing on various aspects of the war (estimated to be almost as much literature as the Second World War!) so there's a book for almost any niche, if you need more specific information or are fascinated by a particular aspect.