The Secret Peace is the fourth young adult adaptation of The Young Indiana Jones Chronicles published by Random House. Released in 1992, the book adapted the episode "Vienna, March 1917".
Publisher's summary
One tough job!
Wanted: Spy to cross enemy lines, escort two princes in disguise to Austrian emperor in Vienna.
Object: Obtain secret letter for French Prime Minister.
Goal: Early end to World War I.
Obstacles: Reckless traveling companions, enemy spies, secret police, ruthless kidnappers, etc.
Requirements: Speak French and German. Work and play well with others. Must be cunning, fast on feet, good driver. Strong nerves a definite plus.
Only one person can do this job: Young Indiana Jones!
Young Indiana Jones—you've seen him on TV in The Young Indiana Jones Chronicles, traveling the world as a kid and a teen, encountering some of the best minds and biggest events in history. Now read about Young Indy's espionage adventure based on the television episode "Vienna, March 1917"!