Shell Shock and its Lessons by Grafton Elliot Smith (1917) [eBook]
World War I Reference Library edited by Sara Pendergast (2002) [eBook]
Links to Diaries, Letters, Photographs, Speeches, Government Documents, Interviews, Paintings etc.
Newspaper Pictorials: World War I Rotogravures (Library of Congress)
WWI Document Archive -- includes extensive links to other websites
The Atlantic: World War I in Photos -- Extensive collection of photographs gathered by Alan Taylor from The Atlantic. Divided into 10 chronological sections. Great resource for photos.