Tightrope by Simon Mawer

After surviving wartime imprisonment, former SOE courier Marian Sutro returns to postwar Britain to find ordinary life impossible. As the Cold War dawns, she is drawn back into clandestine work, entangled with old allies, dubious lovers, and competing intelligence services seeking atomic secrets. Haunted by trauma yet driven by conviction, she walks a perilous line between loyalty and betrayal, autonomy and manipulation, until her choices threaten to destroy the fragile stability she longs to reclaim.