The League of Frightened Men by Rex Stout

It is almost certain that Paul Chapin is a murderer. His former classmates at Harvard are almost certain of it. They are certain he is responsible for the deaths of two of their number, that the oddly worded pages of prose sent to each of them is a proof of his crime. The problem is they have no solid evidence and each is afraid he will be the next entry in Paul Chapin’s list of murder victims.

Enter Nero Wolfe a detective nearly as famous for his love of food, his enormous collection of orchids, and for the fact that he rarely leaves his home as his is for his powers of deduction. But will his skills be enough to save the remaining members of the League of Frightened Men.