I thought it would be a good idea to start a thread to collect sources that provide free available material.
Unlike the US Civil War or Napoleonic eras we will surely have much less publications that are free available because the copyright has still not expired, nonetheless there is material available.
"Der Weltkrieg 1914 bis 1918"
16 volumes providing the official German history of WW1, you can download them but as PDFs they are HUGE, I recommend to browse them online.
"The First World War and the End of the Habsburg Monarchy"
A new book but for what ever reason free to download from the publisher.
This book is a reworked and expanded version of "Der Tod des Doppeladlers" from 1993 that was at its time already a milestone. This new version will surely top that.
If anyone has other material please post it and I add it to this post.