16 books • 4 series
Book #1: Prince of Spies
Book #2: Sea of Spies
Book #3: Ring of Spies
Book #4: End of Spies
Book #1: The Best of Our Spies
Book #2: The Swiss Spy
Book #3: Vienna Spies
Book #4: The Berlin Spies
Book #1: Agent in Berlin
Book #2: Agent in Peril
Book #3: Agent in the Shadows