Restless : a novel / William Boyd.

Enhanced descriptions from Syndetics:

It is Paris, 1939. Twenty-eight year old Eva Delectorskaya is at the funeral of her beloved younger brother. Standing among her family and friends she notices a stranger. Lucas Romer is a patrician looking Englishman with a secretive air and a persuasive manner. He also has a mysterious connection to Kolia, Eva's murdered brother. Romer recruits Eva and soon she is traveling to Scotland to be trained as a spy and work for his underground network. After a successful covert operation in Belgium, she is sent to New York City, where she is involved in manipulating the press in order to shift American public sentiment toward getting involved in WWII.
Three decades on and Eva has buried her dangerous history. She is now Sally Gilmartin, a respectable English widow, living in a picturesque Cotswold village. No one, not even her daughter Ruth, knows her real identity. But once a spy, always a spy. Sally has far too many secrets, and she has no one to trust. Before it is too late, she must confront the demons of her past. This time though she can't do it alone, she needs Ruth's help. Restless is a thrilling espionage novel set during the Second World War and a haunting portrait of a female spy. Full of tension and drama, emotion and history, this is storytelling at its finest.