Code Name Verity by Elizabeth Wein
I put off reading this book for some time despite it sounding perfect for me because I adored Wein's The Winter Prince for about 3/4 of the book, and then in the space of about 3 pages, that love turned to hate and I'm still not over that.
Thankfully, I didn't have the same experience with Code Name Verity set in both France and England during WW2, our narrator is a Scottish SOE wireless operator, codenamed "Verity" who has been captured and is being interrogated by the Gestapo. She's allowed to live as long as she cooperates, and has been given an unlimited supply of paper on which to record everything she knows about SOE operations, and the majority of the narrative is what she writes. Verity chooses to relate what she knows by chronicling her friendship with her pilot, Maddie, who died in a crash (and whose identity papers she's carrying, though she claims not to know how she ended up with them) bringing Verity to France, interspersing her narratives with descriptions of what the Gestapo is doing to her during her imprisonment, and describing what she witnesses happening to other prisoners. I find it a bit farfetched that her interrogator would let her would let her discuss the torture and spend so much time discussing her relationship with Maddie and a couple other female operatives and work in the details he wanted that way, but as a narrative device, particularly one in a plot where an unreliable narrator is crucial, it's an excellent choice.
Wein writes with an extremely strong voice, and much of the tension in the novel (aside, of course, from finding out whether or not Verity will survive) is trying to discover how much of what she writes is true, and how much isn't. It's unsurprisingly a very emotional novel, though it never quite crosses into melodrama, and much of the emotion revolves around Our Heroine's investment in her relationships with other women.
Wein largely avoids specific real names and places in her details about the SOE, but from what I've been reading on the SOE in recent months (Wish Me Luck: Find it. Watch it. Have your brain get devoured by it.) it's very well researched, though there are a couple things that made me raise my brows in doubt. (I'm pretty sure not even the SOE would have sent out an agent who had Verity's reaction to the torture training, or use the methods she claims were used on her during it.) I should also warn that Verity goes into detail about both the torture she suffers through and what she witnesses happening to other prisoners. That and a few other things (among which is all the characters being older than most YA protagonists, though I can't recall if exact ages were ever mentioned) make me a bit surprised that the book is marketted as YA.
Very much recommended if you like ladyspies and/or WW2 stuff not about soldiers in battle and/or interesting narrative choices and don't mind getting figuratively punched in the gut a lot by your fiction.