Diary Of A Provincial Lady In Wartime is one of my favourite books. It's about a hapless London socialite in 1940. Pigeon Pie by Nancy Mitford goes one better and adds spying!
Lady Sophie is a ditzy aristocrat who, in between oysters and gossip, stumbles across a spy ring. When she finally realises what's going on, no-one takes her seriously, so she decides she's just going to have a jolly good go at sorting the spy lark out herself. Oh, and be in time for cocktails with Randolph.
After the war, Mitford distanced herself from it. It's politically dubious and was immediately dated - 2 days after publication, the Phoney War ended - but oh, it's funny!
Features the following lovely put-down of the ghastly evangelist Florence: "Sophie wished that Florence wouldn't talk about God as though his real name was Godfrey, and God was just her nickname for him"