This bulk of this series is composed of incoming and outgoing correspondence. The topics of the letters are mainly literary and religious. Some subjects addressed are: Catholicity in Russia, Church architecture, sacred heraldry, difficulties of Catholic Indians on Government Reservations, and the religious aspects of the issue of vasectomies. Most of the correspondence is regarding contributions to American Ecclesiastical Review and The Dolphin.
One of the notable correspondents is Katharine Drexel (1858-1955), the founder of the Sisters of the Blessed Sacrament dedicated to addressing to the needs of Native Americans and African Americans protesting the injustice of racism. Born to a family which owned a banking fortune, Drexel used her wealth to fund missions and schools. She was beatified by Pope John Paul II on November 20, 1988 and canonized on October 1, 2000. The correspondence is from 1907-1930 and are type or handwritten and includes questions about the new constitution for her order, requests for Latin translations, gratitude for well wishes while she was ill and thanks for his feastday greetings. Other correspondents include American realist painter Thomas Eakins (1844-1916), poet and essayist Louise Imogen Guiney (1861-1920), author and abolitionist Thomas Wentworth Higginson (1823-1911), Associate Justice of the Supreme Court of the United States Oliver Wendell Holmes (1841-1935), essayist Agnes Repplier (1855-1950), photographer and social reformer Jacob Riis (1849-1914), and conductor Leopold Stokowski (1882-1977).
The Caroline B. Perot Letters contain incoming and outgoing correspondence between Heuser and Perot between 1909-1933. The two friends generally discuss personal issues, health concerns and visits. An item of note is a letter dated February 2, 1929 Heuser writes about the author and hymnist Emily Mary Shapcote and how her work, Mary, The Perfect Woman, appeared serially in The Dolphin from 1902-1904.