Anyone who has read and loved Dodie Smith's I Capture the Castle needs to read
The Account Book immediately. Do not pass Go, do not collect $200. Just read at your first opportunity. It's the most perfect story for this fandom that I could possibly imagine - nice and long and meaty, too, and written like it could be another section of the novel. It's Cassandra during the War, and if you don't think that sounds awesome, then what's wrong with you?! Also, there's a pitch-perfect voice and a lot of other familiar faces and some laughs and a nice bit of wish fulfillment at the end.
(And if you've never read I Capture the Castle - well, consider it highly recommended! English family living in reduced circumstances in a ramshackle castle during the 30's; what's not to like? So read that and then, when you're left wanting more, go read this story too.)