Contains sermons, autobiographical notes, a diary, commonplace books, photographs, a scrapbook, memorabilia, and printed matter of Walker, with the bulk of the material comprised of personal and professional correspondence, much of it covering the period of the Civil War. Most of the correspondence consists of letters between Walker and his mother (when Walker was at Harvard), Mary A. Walker, his sister, Mollie, and his wife, Mary, as well as letters from his brother, William Augustus Walker, who was killed in the Civil War, and other family members. As a founding partner in the Boston publishing firm of Walker, Wise, and Company, Walker corresponded with many of the intellectual and literary leaders of the day, such as William Cullen Bryant, William Lloyd Garrison, Julia Ward Howe, Harriet Martineau, Charles Eliot Norton, and Henry James, father of Henry and William James. His firm was also associated with the American Unitarian Association, and the correspondence reflects the publishers' commitment to publish the most liberal Unitarian clergyman and writers of the day, such as James Freeman Clarke, Orville Dewey, Octavius B. Frothingham, and Edward Everett Hale.

In addition, there are manuscripts of Walker's children's sermons delivered as a Unitarian Church lay preacher, photographs of Walker and William Augustus Walker, commonplace books in which he listed books read, plays and concerts attended, and libraries and museums visited, and a scrapbook collection of his writings for the press.