Code Name Verity by Elizabeth Wein

Code Name Verity takes place during WWII.  The protagonist is a female British spy being interrogated by the Nazis.  However, much of the story takes place in flashbacks as the spy recounts for the Germans everything she knows.  Despite this act of cowardice, the reader grows to love and sympathize with the protagonist as she faces choice after impossible choice.  We also grow to love Maddie, the spy's closest friend, as we get to know her through the flashbacks.

This was a great story of bravery, courage, friendship, and war, that I very much enjoyed, and think you would too.