The Smithsonian Institution has created The Great War as a small selection from its digital library. The 28 digitized books cover a wide variety of subjects, from military handbooks, books about the war industry and aircraft to the famous war cartoons of Louis Ramaekers and music. These books were mainly printed in the USA and the United Kingdom.
The digital library of the Smithsonian Libraries contains some 27,000 digitized books on a total of 1,5 million books, and 400,000 smaller publications (ephemera, pamphlets, etc.). You can browse in the digital library for subjects of books. For the subject World War 1914-1918 you will find some 90 digitized books. There is a list of thematic digital book collections.