The Chain of Chance (Stanislas Lem, 1975)

A former astronaut is hired by a detective agency to help in an investigation of a case of mysterious deaths. Several victims became mad and committed suicide during their vacation in Napoli, apparently without reason. Due to certain similarities in the circumstances of the deaths the case is assumed to be a serial murder by poisoning, although it is never certain what (if any) real connection exists between the victims.