This one is my favorite in this series so far, but I'll likely say that about the next one as well. I'm a little disappointed that this one sticks to one person's POV, Jerry Burton in this case.
It's a little odd that we, the readers, never learn what the letters say, but enough is said that we can make a guess. The first letter that the Burton's receieve suggests that they aren't really siblings (they have different coloring), but lovers. The other letters seem to have the same theme, sex in some form.
This one also has a bit of a romance. It's not much really, and I expected it long before it happened. Poor Jerry was completely blindsided by it, which amused me and his sister, Joanna.
Once again, Miss Marple is way ahead of me and I didn't even try to keep up. It's all so simple when she lays it out, but I know better then to try to guess.