Saint Francis
This should not be the first book you read about St. Francis. It is a book for those who are already familiar with the saint. It is a novelization, or fictionalization of the adult life of St. Francis, as told through the eyes of a fictional Brother Leo.
The story is well woven by Kazantzakis, a prolific writer who spent most of his life (1883-1957) in Greece, but who gave up his Greek Orthodox faith at some point in his life (and did not embrace any other faith), who was introduced to St. Francis when he visited the city of Assisi.