The Gilder mss., 1781–1984, consist of correspondence and papers of poet, editor Richard Watson Gilder and his wife, the artist Helena de Kay Gilder, and their family. In addition to family members, the correspondence reflects the many friends and acquaintances who visited the Gilders at their Tyringham, Massachusetts home, as well as their private salon in New York. These include artists, musicians, writers, poets, scientists and politicians, such as August Saint–Gaudens, Winslow Homer, Mark Twain, and Nicola Tesla. There is extensive correspondence with Frances Folsom Cleveland (Mrs. Grover Cleveland), artist Cecilia Beaux and writer Mary Hallock Foote. The Writings series contains works of Richard and Helena, as well as many other writers and poets, including Phyllis de Kay Wheelock who wrote about the De Kay family, and extensive files of Rosamond Gilder's writings and research. Writings by William Henry Gilder (1838–1900) include articles on the Arctic and a description of his encounter with pirates. Many pencil and watercolor sketches and sketchbooks of Helena may be found in the Artwork series. A few other artists are represented, mostly family members, but include a few sketches by Mary Hallock Foote. An interesting joint diary of Helena and Richard, dated 1874–1888, may be found in the Diaries/Journals series. The Legal materials mostly concern estates and properties. Formal portraits of the Gilder family, friends and acquaintances, as well as snapshots taken on their various travels make up the Photographs series.

The Miscellaneous series contains military–related items, including a report to Headquarters by surgeon Reuben Gilder concerning the dead and wounded from the Battle of Cowpens in 1781, a group of letters picked up from several Civil War battlefields by Richard's father William Henry Gilder, and paperwork reimbursing Richard's brother William Henry Gilder for the death of his horse at the Petersburg fight in 1864. From Richard's brother William Henry's Arctic explorations is a list of provisions, list of backers, map, and various printed items. There is extensive ephemera in the Miscellaneous series: concert and lecture programs, invitations to various events, brochures, maps, and menus. Realia of interest include: a lock of Helena's hair, dried flowers from John Keats' grave picked in 1884, a pencil used by Walt Whitman, and dried leaves collected in 1863 from the Bull Run battlefield.