Monsignor R. Fontenelle's His Holiness Pope Pius XI is a biography that offers in-depth insight into the life and pontificate of Pope Pius XI (Achille Ratti), who reigned from 1922 to 1939. It chronicles Achille Ratti's life from his childhood, highlighting his brilliance as a scholar, paleographer, and prefect of the Vatican Library, before assuming the highest office in the Catholic Church. Fontenelle portrays Pope Pius XI as a man of great academic, political, and spiritual prowess. His influence is described as transcending geographical boundaries, leading to a congregation of some 350 million Catholics during his time. He published important encyclicals such as Quadragesimo anno (1931), which outlined a comprehensive vision of Catholic social teaching on economic inequality, labor rights, and the role of the state based on Christian principles. Overall, the book provides a comprehensive and honest portrait of a spiritual leader facing enormous challenges in a turbulent time, highlighting the character and enduring legacy of Pope Pius XI.