The Fort Branch Battlefield Commission has published a pair of monographs, the sales of which benefit the Fort Branch site preservation.
Martin County During the Civil War by James H. McCallum, M.D. and Fort Branch And The Defense Of the Roanoke Valley 1862 - 1865 by Phillip Shiman look to be of value, and are very reasonably priced to boot (esp. when purchased together). The latter is a brief military summary of the fort's history that also offers a number of maps and illustrations (incl. a highly detailed pull-out historical map of the surrounding area).