Collection contains letters sent between William Foster Jr. and William's father, siblings and their spouses, nieces and nephews, and children. Correspondents include brother-in-law Harrison Gray Otis, Andrew Ritchie, daughter and son-in-law James E. and Virginia (Foster) Coppinger, daughter Jeanne Francoise "Fanny" (Foster) Tudor, brother Leonard Foster, sisters Grace (Foster) Apthorp and Mary (Foster) Apthorp, and Nathan Spear Jr. William also received personal letters from friends in Cadiz, Spain and his wife's relatives in France. The political correspondence includes letters and treatises written for publication in newspapers and pamphlets as well as copies of letters sent to prominent political figures by William Foster Jr and their replies. William wrote to all the leading figures of the day, especially Anti-Masons and Democrats. Correspondents include John Quincy Adams, John A. Andrew, Thomas Hart Benton, Lewis Cass, Caleb Cushing, James C. Dobbin, Alexander Hill Everett, Edward Everett, Levi Lincoln, Marcus Morton, Franklin Pierce, John Randolph, Charles Sumner, John P. Van Ness, Daniel Webster, and Silas Wright. William's many schemes for public improvement led him to correspond with Peter Cooper and with men involved in forming the Smithsonian Institution. There are legal documents, both in English and French, membership certificates and some brief reminiscences relating to William Foster Jr.

The collection also includes letters written by Harrison G. and Sally (Foster) Otis, Grace Jeanne Melite Foster, Virginia (Foster) Coppinger, William Leonard Foster, Charles Chauncy Foster, Francis Charles Foster, and William Leonard Foster to various family members besides William Foster Jr. There are papers concerning the family real estate holdings, held in common, that centered on Spear's Wharf (North End) and Foster's Wharf (foot of Fort Hill). The Fosters also had interest in land on Fort Hill and the South Cove areas of Boston. The material includes ledgers from scattered years concerning Foster's Wharf receipts, and a great deal on various municipal land takings for improving markets and widening streets. There are plot plans and maps for areas of Boston in which the Fosters had an interest. In addition to the correspondence, the collection contains ten scattered years of William Foster Sr's diary, patents by Leonard Foster, portrait miniatures, and photographs including daguerreotypes and ambrotypes.