The following suggestions for further reading are from my Unlikely Warriors:
For British politics in the 1930s, see John Stephenson and Chris Cook’s Britain in the Depression and Juliet Gardiner’s The Thirties. Joe Jacobs’ memoir Out of the Ghetto is good for a view from the street.
There are many published accounts by British volunteers in the International Brigades; of those still in print, Walter Gregory’s The Shallow Grave and Fred Thomas’s To Tilt at Windmills are justifiably popular. George Orwell’s Homage to Catalonia and the Spanish born Arturo Barea’s The Forging of a Rebel are both important and highly readable. Many works focus on the national and ethnic groups within the ‘British’ volunteers, of which Hywell Francis’ reissued Miners Against Fascism and Daniel Gray’s Homage to Caledonia are two notable recent additions.
Paul Preston’s We Saw Spain Die is a fascinating account of the foreign correspondents who witnessed the conflict. For the war itself, Hugh Thomas’s The Spanish Civil War in its fully revised third edition is always useful, though Helen Graham’s The Spanish Civil War and Paul Preston’s Concise History offer more accessible introductions to the subject. Ronald Fraser’s Blood of Spain remains a shining example of the merits of oral history.
For the role of the former volunteers after the civil war and the continuing relevance of the conflict, see Tom Buchanan’s Impact of the Spanish Civil War on Britain.