There are some invaluable resources for studying the Great War on the Internet. This list is limited at the moment but I’m just getting started.
The War Diaries of French units from army groups to sanitation companies are available as high resolution scanned images on Memoires des Hommes. This site, more than any other, has allowed me to carry on the research I do.
The proprietor of this site has transcribed the personal diaries (called “carnets”) of over 140 French soldiers of the Great War. There are also useful links to other documentary resources.
This site contains French regimental histories in digital format from diverse historical periods.