The George C. Marshall Foundation's digital library has a number of valuable resources.
The on line edition first five volumes of Marshall's papers are available at the following URLs. One should note that each paper is a separate Microsoft Word document, and each document has a suggested reference. However, I believe the references conform to a previous edition of the
Chicago Manual of Style rather than the more recent sixteenth edition.
Volume 1: The Soldierly Spirit, December 1880 - June 1939
Volume 2: We Cannot Delay, July 1, 1939-December 6, 1941
Volume 3: The Right Man for the Job, December 7, 1941-May 31, 1943
Volume 4: Aggressive and Determined Leadership, June 1, 1943-December 31, 1944
Volume 5: The Finest Soldier, January 1, 1945--January 7, 1947
Marshall's congressional testimony is available
here.
Forrest C. Pouge's biography of Marshall is available at the following links.
Volume 1: Education of a General
Volume 2: Ordeal and Hope
Volume 3: Organizer of Victory
Volume 4: George C Marshall Statesman
A collection of Pogue's interviews and recollections are
here.