Code Name Verity by Elizabeth Wein

Description from Goodreads: "A British spy plane crashes in Nazi-occupied France. When 'Verity' is arrested by the Gestapo, she's sure she doesn't stand a chance. As a secret agent captured in enemy territory, she's living a spy's worst nightmare. Her Nazi interrogators give her a simple choice: reveal her mission or face a grisly execution. As she intricately weaves her confession, Verity uncovers her past, how she became friends with the pilot Maddie, and why she left Maddie in the wrecked fuselage of their plane. On each new scrap of paper, Verity battles for her life, confronting her views on courage, failure, and her desperate hope to make it home. But will trading her secrets be enough to save her from the enemy?"

Watch the book trailer on YouTube.