Although Father Roger Riou, the tough missionary priest of, until recently, the Haitian island of Tortuga, had some years ago been dubbed ""the Dr. Schweitzer of the Caribbean,"" there are striking dissimilarities in philosophy and background. Fr. Riou, born in 1909, spent his childhood among whores, pimps, and exploited workers in the infested slums of Le Havre. As for schooling, ""We were too poor, too battered by life for me to have an unselfish taste for learning."" His overworked mother died in a hospital because he could not afford to pay a doctor. During his term in a reform school he discovered in the Gospels ""a Christ open to the misery of men. . ."" and the thought that he would like to be a priest came out ""just like that--without thinking."" Fr. Riou details his rigorous and difficult studies for the priesthood at age twenty-two and his initial orientation to missionary work in the appalling poverty and disease of Tortuga. Although on the mainland Fr. Riou assisted in an anti-voodoo campaign, he constantly stresses the need to work with native customs, and his primary concern was providing the basic necessities of life for his people, ""to make them feel they were not utterly rejected."" Gradually, painfully, help came from the outside, and the dispensary grew from what was at first a line of wretched people waiting for injections of distilled water (the only ""medicine"" the priest had) to a small but reasonably well-staffed and supplied hospital. He stayed until the henchmen of Papa Doc, enraged because they were not getting their cut from foreign charitable organizations, drove him from his post. Fr. Riou is a feisty little cleric: he tells of seminarians beating up hecklers with great relish; he has been known to use shouting in lieu of the anesthetiche lacked; he is outspohen about certain autocratic church practices and officials in the Vatican (""a narrow.minded clan""). Fr. Riou is no stylist--and one suspects a tendency to decorate fact--but the poivre makes up for the lack of politesse.