'Casino Royale' by Ian Fleming is also highly popular. It introduced the world to James Bond in a thrilling spy adventure. The high - stakes gambling and Bond's suave yet dangerous character made it a classic.
'Alias Grace' by Margaret Atwood has a strong following too. While it may not be a traditional spy novel in the sense of international espionage, it has elements of mystery and intrigue within a historical context that many spy novel fans enjoy.
One of the most popular is likely 'The Bourne Identity' by Robert Ludlum. It has been extremely successful and was made into a popular movie franchise. The story of Jason Bourne, a man with amnesia trying to figure out his past as a spy, is very engaging.