Wein, Elizabeth. Code Name Verity
The winner of several awards including Library Journal's Best YA Lit for Adults 2012, Code Name Verity is the story of a friendship that remains strong through the dangers and upheavals of WWII. A female British spy is captured in Nazi-occupied France, tortured and forced to reveal all the information she knows. Her writings become an enthralling account of the woman whose identification card she happened to be carrying, her best friend, Maddie. Together, they ended up working for the British war effort, although in two very different ways, one as a transport pilot and the other as an intelligence officer. The narrator remains cryptic until the story switches to Maddie's point of view and we find out she is closer than her best friend thinks and trying her hardest to find her.
This tale is fascinating and keeps up a good pace all the way through. If you have the chance, check out the audiobook. One narrator especially brings the story to life with her soft Scottish accent.