The Chain of Chance - a novel by Stanislaw Lem (Polish copyright 1975.) English translation by Louis Iribarne (1978.) Since I haven't read this, here's the blurb from the back cover of a 1979 paperback, followed by the flap copy from a hardcover:
"WAS IT COINCIDENCE - OR CONSPIRACY? At a seaside luxury resort a dozen tourists suddenly vanish. Go mad. Or die. All of the victims are single, mature, athletic males. To crack this baffling code of death, an American astronaut sleuth begins to tinker with the laws of chance. But before he can penetrate the infinite riddle, he is threatened by an airport bomber, stalked by a potent chemical danger, and menaced by nature's own timeless rules - as they seem to turn inside out - and a psychedelic netherworld, slowly, inexorably, claims him."
[flap-copy from the hardcover]:
A series of mysterious deaths baffles the Italian police. At a seaside resort in southern Italy, a number of tourists - all of them middle-aged men and foreigners - vanish, go mad, or die. What is the plot behind this pattern? A former astronaut turned private investigator is dispatched from the U.S. to unravel the mystery, which he tries to do by setting himself up as a decoy. Just as he is on the verge of giving up in despair, violent events happen. But the pattern of cause and effect reveals itself only through a series of extraordinary yet fully credible coincidences...