I strongly recommend Anthony Beevor, the battle for spain
Ditto from me! A great read! 
Thanks, I have it. I'd say it was the best general read on the War, with 2nd and 3rd place going to;
The Spanish Civil War - Paul Preston
The Spanish Civil War - Hugh Thomas
La Batalla del Ebro - Jorge M. Reverte (Excellent in-depth study of the campaign, particularly if your Spanish is good). Reverte has done one on the Siege of Madrid too, I don't have that one so can't comment.
Between Two Fires - David Baird (Deals with mainly the post-war guerrillas operating in Andalucia against the Franco Government, some excellent ideas for skirmish games and a chance to use your Guardia Civil).
Another good read is 'Ghosts of Spain' by Giles Tremlett. Although it is not a SCW per se, it deals with how the Spanish are coming to terms with dealing with the war's legacy and the Franco government's suppression of discussion. I thoroughly enjoyed this book, very human and very orientated to an outsider's view of the Spanish, so you don't need to be an expert on Spanish history to follow it.
'Guerra' by Jason Webster is a similar book and concentrates on the legacy of fascism and the war, starting with the discovery of a mass grave on the farm he'd bought, his neighbour's recollections of the event (her family were some of the victims) and branches out a little from there. Not as good as 'Ghosts' and quite dark in it's outlook in places, but worth reading nonetheless.