I did not write up any of the books I was reading up at the time, under the general Yuletide veil of secrecy, but I think all of them are worth the perusal:
Agent Zigzag: A True Story of Nazi Espionage, Love and Betrayal, Ben Macintyre
Like most of Ben Macintyre's books, this is a true-history spyjinks story which leans heavily on the hijinks. Eddie Chapman was a thief in prison on the Channel Islands when they were occupied by the Germans; he got the bright idea to get out of prison by offering his services to the Germans as a spy. Then, after being thoroughly trained in Advanced Spying by the Germans, he was parachuted into England to blow up a factory, where he was immediately caught by the British, and promptly informed them that he was in fact more than happy to be a double agent.
The usual sort of Elaborate MI5 Ruses followed, perhaps the most impressive being the hiring of a stage magician to fake the destruction of an entire factory for the Germans so they would assume Chapman was still a totally independent Nazi agent, yes sirree. It's not my favorite of Macintyre's books, but it's a fun read -- or it was at the time; I suspect "lol, those gullible folk-dancing Nazis!" might seem a bit less funny now that we are all realizing how very much Nazis are no longer a thing of the past.
(Ben Macintyre's funny bone is clearly tremendously tickled by the Nazi Who Obsessed Over English Folk Dancing. He never misses an opportunity to bring it up. Long after I have forgotten the rest of this book, I will remember the English folk-dancing Nazi.)
Nella Last's War: The Second World War Diaries of Housewife, 49
So Mass Observation was a research organization founded in the 1930s which encouraged Ordinary British Citizens to write in or diary about their daily lives, which quite by happenstance resulted in the creation of this really astoundingly thorough primary-source record of what it was like for a middle-aged British woman to live through WWII. In other words, a researcher's godsend.
It's also sort of astounding how much of a plot there is to this unstructured diary; it feels like something that could be a novel. Nella Last, at the beginning of the war, is a housewife married to a man who doesn't much like to go out and doesn't much like for Nella to go out either, at all, ever. As the book goes on, and she starts taking on war work and becoming involved in local organizations, she begins to write more and more about how trapped and stifled she's felt for most of her marriage; she starts standing up to her husband, taking on new projects, sleeping downstairs in the bomb shelter just so she can have her own space. And meanwhile one of her sons has to join the army, and hates it, and ... falls in love with another soldier? ... I mean obviously Nella Last doesn't say or think that that's what it is, and I am hesitant to start writing RPF about ordinary people, but it looks an awful lot to me like that's what is going on. Fiction has its patterns for a reason, is I guess what I'm saying.
Anyway, it's a fascinating read, though generally not a cheerful one. And occasionally some bit of period-specific awfulness of Nella's will come up and hit you in the face -- when she chattily goes on for a while about how obviously Hitler is awful but perhaps he's not entirely wrong on the eugenics thing, for example, or when her other son comes home and starts complaining about the Jews in his town and Nella's like "lol kiddo looks like you've gotten a bit racist!" in the most unconcerned fashion imaginable.
A Life in Secrets: Vera Atkins and the Missing Agents of WWII, Sarah Helm
This is the book which really ended up shaping my Yuletide fic the most -- like, the thing I wrote is probably based on this book as much or more as it is on The Bletchley Circle.
Vera Atkins, a Jewish Romanian, joined the British SOE division as a secretary in 1941. She quickly rose in the organization, became the head of section F (France) and became influential in the deployment of women agents behind the lines. In 1943, the primary network of British agents in France was compromised; though many of the men and women who were captured by the Germans tried their hardest to alert headquarters, SOE's refusal to believe anything was seriously wrong meant numerous other agents heading to France dropped straight into Nazi captivity. This is the grim flipside of Macintyre's trademark cheery spyjinks.
(One captured agent radioed in under duress and pointedly did not provide his double-secret security code -- the whole point of the double-secret security code was to show when someone was radio-ing under duress -- and Atkins' boss radioed cheerily back to tell him that he'd forgotten his double-secret security code and not to do it again! I MEAN.)
Many of Vera Atkins' agents turned up after the war, but many more did not. This book is only partly about the actual wartime espionage; much of the rest of it is about Vera Atkins' determined journey across postwar Europe, visiting concentration camp after concentration camp to attempt to find out what happened to the missing ones. As you might imagine, this does not make for easy reading. But at least her quest wasn't fruitless; she did eventually trace every last one of them.
(For the record, there also exists a RIVAL biography of Vera Atkins. I did not read it, but there is a beautifully scathing review of it that purports to be from the author of this biography, which you can read here if, like me, you are entertained by the prospect of historians getting into fistfights over their subject matter.)
I also read the parts of Spy Princess: The Life of Noor Inayat Khan that were available via Google books, because I could not get my hands on a copy in time to read the whole thing before I had to write my fic. But the bits I could read were extremely helpful and I do intend to read the whole thing at some point! Noor Inayat Khan also turns up quite a lot in A Life In Secrets; Sarah Helm seems to think that Vera Atkins was particularly interested in Noor among all her agents, but personally I think this may just be due to the fact that Sarah Helm was particularly interested in Noor among all Vera's agents. Not that one can blame her -- her story is tragic, but incredibly compelling.