Code Name Verity by Elizabeth Wein

Wein, E. (2012). Code name Verity. New York: Hyperion.
That's what you do to enemy agents. It's what we do to enemy agents. But I look at all the dark and twisted roads ahead and cooperation is the easy way out. Possibly the only way out for a girl caught red-handed doing dirty work like mine - and I will do anything, anything to avoid SS-Hauptsturmführer von Linden interrogating me again. He has said that I can have as much paper as I need. All I have to do is cough up everything I can remember about the British War Effort. And I'm going to. But the story of how I came to be here starts with my friend Maddie. She is the pilot who flew me into France - an Allied Invasion of Two.  We are a sensational team.  What happens to spies is that they shoot them.  Maddie grew up with her grandparents and was a very good mechanic. She learned to fly when she fell in love with Dympna who was an avid pilot. After joining the Women's Auxiliary Air Force, Maddie became friends with Julia who was a British spy. They were flying on a mission when the tail was hit and Julia parachuted out.  She was captured by the Gestapo. After crash landing, Maddie stayed with the Resistance.  The Gestapo wanted Julia to give them information by torturing her, but all she did was write about her friendship with Maddie. The French Resistance heard that the prisoners were being moved, but they had no way of helping Julia escape.  So that Julia would not be tortured any more, Maddie shot her friend.  Julia's mother then received the letters that both girls wrote.  This World War II novel is about sacrifice, friendship, and trust.  The follow up book to Code Name Verity is Rose Under Fire.