Winter in Madrid by C.J. Sansom

From the bestselling author of the Shardlake series, C. J. Sansom, comes Winter in Madrid, a standalone historical novel set in the aftermath of the Spanish Civil War.
1940: The Spanish Civil War is over, and Madrid lies ruined, its people starving, while the Germans continue their relentless march through Europe. Britain now stands alone while General Franco considers whether to abandon neutrality and enter the war.
Into this uncertain world comes Harry Brett: a traumatized veteran of Dunkirk turned reluctant spy for the British Secret Service. Sent to gain the confidence of old school friend Sandy Forsyth, now a shady Madrid businessman, Harry finds himself involved in a dangerous game – and surrounded by memories.
Meanwhile Sandy's girlfriend, ex-Red Cross nurse Barbara Clare, is engaged in a secret mission of her own – to find her former lover Bernie Piper, a passionate Communist in the International Brigades, who vanished on the bloody battlefields of the Jarama.