As fascinating as it was to read a book about pilots in the RAF during World War II, I do wish the author had recognized the difficulties inherent in spewing large batches of data about planes and codes and airports at her readers. Without advancing the story at the same time.
I'll say right off the bat that the last 1/4 of this book moves very, very quickly. The plot twists were achieved persuasively, and there were scenes that did make me tear up, they were so emotional. However, the artificiality of telling a story via a batch of letters makes it a risky endeavor for any author. You have to be very, very good at what you do to pull that off. (Although I can think of at least 3 more books that have done that recently, so maybe it's commonly done now...?)
In the end, the essential boring-ness of the entire first half of the novel makes it way too much of a slow burn. It's war after all - how about some activity? When the friends start describing their fears for the fourth darn time, I was really ready to move on. I can't recommend skipping to the back 1/4 of the book unless you have read at least the first 30%, so it's a bit of a conundrum. I'll stick with not recommending it, sadly.