José Peirats, The CNT in the Spanish Revolution, published by The Meltzer Press (Vol. 1) and Christie Books (Vol. 2). They are essential reading.
Stuart Christie, We, The Anarchists, The Meltzer Press. A telling and thorough look at the FAI.
Much of our knowledge of the anarchist resistance to Franco comes from the work of Antonio Tellez and Miguel Garcia. All of the above publications are available from AK Press akpress.org and deserve close reading.
Also indispensable are the pamphlets and documents available from the Kate Sharpley Library at katesharpleylibrary.net.
There are dozens of other worthy sources as well, but these are a good beginning.
Related: Fifth Estate Spanish Revolution Resource Page