Daniel Silva and Gabriel Allon and I have to mention John LeCarre
My Dad and others have recommended author Daniel Silva as a good read. Here are his books involving secret spy Gabriel Allon and a link to his website:
The Kill Artist
The English Assassin
The Confessor
A Death in Vienna
Prince of Fire
The Messenger
The Secret Servant
The Moscow Rules
When it comes to the secret spy genre, the reigning king has been in my opinion John Le Carre. His books seem to appeal more to readers who are interested in character development, morality and ambiguity, and tragedy. For anyone interested, they should start with The Spy Who Came In from the Cold and the introduction to George Smiley.