The Enigma Game
Image can be found at: https://en.m.wikipedia.org/wiki/File:Four-rotor-enigma.jpg
The Enigma Game by Elizabeth Wein
Text from the back cover:
1940.
Facing a seemingly endless war, fifteen-year-old Louisa Adair accepts a position caring for an elderly German woman in the small village of Windyedge, Scotland. In Windyedge, Ellen McEwen, a volunteer driver with the Royal Air Force, and Jamie Beaufort-Stuart, a squadron flight leader, are facing a barrage of unbreakable code and enemy attacks they can't anticipate. Then a German pilot lands in Windyedge under mysterious circumstances and plants a key that leads Louisa to an unparalleled discovery: an Enigma machine that translates German code. Louisa, Ellen, and Jamie must work together to unravel a puzzle that could turn the tide of the war -- but puts them directly in the crosshairs of the enemy.
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Ms. Sharp's quick review: Sometimes, your enemy is on your side.