As it turns out, it is about an author, Juliet, who begins to correspond with (and later meets, and later becomes one of) the members of the Guernsey Literary And Potato Peel Pie Society, who live on the island of Guernsey, which was occupied by Germany during the Second World War.
I also didn't realise it was written in "epistolatory style", as I since read somewhere. I have read a few books written in letters and email and didn't particularly enjoy them - I think because there is so much telling and not showing, and because while telling long-winded stories it is hard to maintain a character's letter-writing tone. I did enjoy this one, once I had gotten used to the style and started to get all the characters straight.
The final thing I didn't know before I read it was that the author's health problems meant the book was finished by her niece. I didn't notice any major difference in writing style, but maybe I would have if I'd known. I think I like that I wasn't distracted and analysing in that way.
The moment I decided I had to love the book was on page 122, when Juliet sends a telegram to one of the Society's members regarding a recent letter. Up until now they had all only communicated by mail, but clearly this was an event worthy of sending a telegram:
Telegram from Juliet to Isola
23rd April 1946
Did Elizabeth really slap Adelaide Addison STOP If only I had been there STOP Please send details STOP Love Juliet
The book does finish with a happy ending, but not the ultimate happy ending that could have gone down. It was an interesting look at a very small part of the impact of WWII. It was easy to see how the authors (Juliet the character, and the actual author, Mary Ann Shaffer) could become so involved in the lives of the people living on the island.