Start with high-confidence spy stories books, then narrow by subgenre or adjacent topics.
Patriots, Redcoats and Spies
Robert J. Skead
Don't You Know There's a War On?
Avi
Projekt 1065: a Novel of World War II
Alan Gratz
Max in the House of Spies
Adam Gidwitz
Max in the Land of Lies
The Bletchley Riddle
Ruta Sepetys, Steve Sheinkin
Phoebe the Spy
Judith Berry Griffin
Allies
Europa
James Ponti
