The Bletchley Riddle by Ruta Sepetys
In this compelling historical adventure, siblings Lizzie (14) and Jakob (19) contribute to the war effort in 1940s England while also investigating what happened to their mother. Told in alternating points of view, reserved Jakob and precocious Lizzie share how they arrived at Bletchley Park, the epicenter of Allied codebreaking efforts to unlock the German Enigma machine (hint: one was recruited at school and the other evaded efforts to send her to America).