I read them all avidly about 40 years ago and am rediscovering them via Charity shops atm.
One of my favourites was about a group of people living in a shared house, possibly a student house and there were letters written in green ink. I've had a google but can't find the name of the book. Any ideas please?
No spoilers please because I can only remember the murderer in a few and don't want to spoil the rest.