The Priests Madonna By Amy Hassinger

(Used, paperback)
In 1896, a priest in southern France suddenly gained immense wealth. This much is historically true-but no one knows where the money came from. At his death, the priest's secret was buried with him. But there was a young woman-his housekeeper and lover, Marie Denarnaud-who may have known the truth. 
Marie Denarnaud is only a girl when the charismatic priest Berenger first captivates her. As a woman, she becomes his housekeeper and struggles with guilt over their illicit relationship. But as their affection deepens, Berenger grows secretive, and Marie has to learn why. What she discovers is a shocking secret that she becomes responsible for concealing.