I chose George Orwell's Homage to Catalonia as my introduction to the conflict, mostly because it was available at the library on audiobook. While that isn't the most flattering reason to choose a book, I didn't really know what else would work. I didn't want a super-detailed, exhaustive history, and I wanted to read a first-hand account.
Orwell was a pretty great guide. I loved his dry sense of humor. It is hard to imagine Orwell having strong feelings about anything, based on his narrative style, but clearly he felt strongly enough about a cause to go to a foreign country and fight for it. (And, obviously, he felt strongly about many things, based on the subjects he chose to write about.)
I do think many of the intricacies of the politics and maneuverings were over my head, possibly because I was reading this via audiobook and possibly because Orwell assumed that his readers would have at least a passing knowledge of current events at the time of publication. Unfortunately, I have hardly any working knowledge of what was going on during the Civil War, so I was a bit at sea during some chapters. But I didn't mind because the other chapters were very engaging. Orwell definitely falls victim to stereotyping, describing Spaniards as slow and lazy, Italians as fashionable, etc., but he does it with so much humor that it's hard to take it very seriously. He also gives himself the same treatment - at the beginning of the book, he talks about his obsession with learning how to use a machine gun, and using his very limited Spanish skills to ask if he can learn every day. But instead of mastering the weapon, all the soldiers are taught is how to look good in a parade.
But even more than the humor, what stood out in this book was Orwell's own experience in the war. He started as an idealist socialist, but as the war continued and he saw first-hand the effect on both soldiers and civilians, the propaganda machine, the lies and the politics, his perspective changed. He no longer trusted the Communists to be honest and straight-forward; he saw that they, too, lied and cheated and committed all sorts of atrocities. And then, I assume, he went and wrote Animal Farm, which proceeds in much the same manner.
Homage to Catalonia is an excellent read to fully appreciate Orwell's writing style and humor. It's also a very honest look at how ideals can be lost in the midst of a horrible and bloody war. While I don't know if it's the best book to read to get an understanding of the background and lead-up to the Spanish Civil War, it's definitely an excellent book to get you interested in the conflict. And to understand a man's internal conflict, too. Highly recommended.