I'm currently doing an Extended Project Qualification (EPQ) for my A-Levels (UK). My chosen topic is The Spanish Civil War, focusing specifically on what factors lead to the Nationalist victory. I am already using The Battle For Spain - The Spanish Civil War 1936-1939 by Antony Beevor and The Spanish Civil War by Hugh Thomas. If anyone knows any further books, articles or textbooks that apply to my specific topic I would appreciate it.
While you wait for further recommednations, it might be worth checking out this wonderful AMA with /u/crrpit.
There's a decent variety of texts speaking to this question in the AH booklist. Thomas and Beevor are both on there, and are good starting points, but also have their limitations as overviews. Dealing with that is, of course, the point of your assessment.