Internal evidence suggests that the author of these letters, "W. F. Farrington," who worked for the United States Christian Commission at Sickles Hospital in Alexandria, Virginia, in mid-June 1865, is Methodist minister William F. Farrington (1800-1888).
William F. Farrington was born on September 7, 1800, the son of William Farrington and Hannah Freeman, and became a Methodist minister in 1829. Around 1842, he became pastor of Chestnut Street Methodist Church in Portland, Maine; from 1864-1866, he led the Methodist Episcopal Church in East Bridgewater, Massachusetts. Farrington was married three times; his wives included Mary Ann Bolton and Margaret P. Skillings (m. March 25, 1863). William F. Farrington died on December 26, 1888.
From the guide to the W. F. Farrington letters, Farrington, W. F. letters, 1865, (William L. Clements Library, University of Michigan)