The early parts that focused on the formation of the literary society were engaging. I enjoy watching people learning to connect with literature and seeing how people come to identify with different works. I also enjoyed learning a little about the occupation of Guernsey and what the wartime experience was for its inhabitants.
I did find it a little hard to connect with the characters in general, partly because of the epistolary structure and partly because of a somewhat unfocused story arc. The letters themselves seemed to strain reality, with one character regularly rushing off to the telegraph office to send someone the 1940s equivalent of "OMG! I M totes in <3 w/D!" Did people really do that? It may have been necessitated by the epistolary structure, to provide some indication of her thoughts on events as they transpired, but it still felt weird. The false notes seemed more prevalent in the second half of the book than the first, and I couldn't help wondering, rather guiltily, how many were contributed by the niece.
Overall, though, I found it a pleasant enough read. I just doubt I'll remember anything about the characters in a few weeks.