I wrote AyV back in the day, my experience is that there's not a single source you can turn to, much less in Spanish. A good starting point for the OOB is orbat.com that has the peacetime OOB of the Spanish Army, but there's zilch on the net on the militias and the security forces.
Also, shortly after the war kicked off, the Republicans disbanded the regular army for practical purposes. If you are willing to have patience, you can trawl through the threads here http://www.elgrancapitan.org/foro/ although in Spanish, of course. On the book front there are 2 collections in Spanish, the official history (nationalists vs reds...) and the History of the people's army, both are pricey, rare and in Spanish...
From the start I intended to do a 1st part called Alzamiento y Revolución, I did some preliminary work, but then my children were born and it's on hold for another 18 years at least...