Paul Preston's "The Spanish Civil War" would be the poster child of the "quality non-fiction" category specified in this post's title. He is, in my opinion, the leading Hispanist historian [1], and I hold his work in the highest esteem. I can also very much recommend his book "A People Betrayed".
Regarding the other two authors in my previous comment: Hugh Thomas was also a leading Hispanist historian [2], and Antony Beevor is a prominent military historian [3]. I expect both books to be in the same vein as Preston's.
(Quick aside for the astute reader that noticed all of the recommended authors being non-Spanish: I find Spanish politics extremely polarized; the Spanish Civil War being perhaps the most polarizing topic. As such, I find outsider's perspectives to be more balanced and objective.)
Regarding the other two authors in my previous comment: Hugh Thomas was also a leading Hispanist historian [2], and Antony Beevor is a prominent military historian [3]. I expect both books to be in the same vein as Preston's.
(Quick aside for the astute reader that noticed all of the recommended authors being non-Spanish: I find Spanish politics extremely polarized; the Spanish Civil War being perhaps the most polarizing topic. As such, I find outsider's perspectives to be more balanced and objective.)
References:
1. https://en.wikipedia.org/wiki/Paul_Preston
2. https://en.wikipedia.org/wiki/Hugh_Thomas,_Baron_Thomas_of_S...
3. https://en.wikipedia.org/wiki/Antony_Beevor