I'm a huge fan of Daniel Silva's novels. The Unlikely Spy, his first novel, is more uneven than his previous books, as first novels often are (On the Road, for example,) but it's a fun WWII spy vs. spy suspense with great, deep characterization.
Arthur Vicary, a professor, is somewhat impressed into service in intelligence by his friend, Winston Churchill. His enemy, though he does not know who she is or even, at first, that she exists, is Catherine Blake, a deep-cover mole who has been inactive since the beginning of the war. She's also been coerced to serve, though with the ruthlessness one would expect from the Nazis. This makes her an ambivalent villainess, which makes for a far more interesting book than if she were merely a Mauser-toting, stiff-arm-flapping, knee-jerk honey trap.
Here, Silva begins his explorations into the damage that a human psyche must acquire before the person can truly become a spy and a murderer. It's an interesting question more fully explored in Silva's later books about Gabriel Allon.
The plot is a basic one: can the Axis discover whether the D-Day invasion will be at Calais or Normandy, and can the Allies stop them from discovering it? No one ever went wrong with a strong plot.
Because this is not billed as alternate history (like *The Plot Against America* by Roth,) you can kinda figure out the ending. Luckily, this book is about the ride, not about the end point.
TK Kenyon Author of Rabid: A Novel and Callous: A Novel