The League of Frightened Men

Rex Stout

This is the second Nero Wolfe book (1935). From Wikipedia: "When two men die under mysterious circumstances and a third disappears after appealing to Wolfe for protection, suspicion falls on Paul Chapin, a controversial author and friend of the three men who was severely injured many years ago at their hands as a result of a hazing prank. The remainder of the men involved, united together as a "League of Atonement", are in fear for their lives against Chapin's vengeance, but when a third man dies Wolfe determines that he may not be the only threat they face."