I *deeply* enjoyed this book. I think it's the best in the series so far. The tone of these books is still less historical than I'd personally prefer, but this one required less suspension of disbelief for me.
First off, I LOVED the hook of this story, the letters, and I loved that both the heroine and the hero had really different reasons for a) carrying on the charade of the relationship during the letter period and b) eventually going through with it and getting married.
LOVED that the ridiculousness of it got lampshaded by the aunt being in on it the entire time and loved that the heroine's terrible fear of crowds was explained but not lingered over too much. Liked that there wasn't a magic fix, and the ridiculousness of them telling each other that no, they were actually in love with them wasn't carried out tooooooo far.