The Sarah Poppen De Koster papers date from 1896 to 1932, and measure 5.00 linear inches.

The Biographical series (1896-1932) contains letters to De Koster, most written during World War I. Many letters are from De Koster's brother John, who served in the war as a medical officer and struggled with loneliness and family obligations. Life during the war is further reflected in a U.S. Food Administration letter on sugar rations as well as correspondence from other friends at home. Also included are wedding and newborn announcements and personal correspondence.

Completing the collection is a family retrospective by State Senator Lucas De Koster, Sarah Poppen De Koster's son, called The Books of Our Years.