This is a list, which I will update as I can, of sources for files for the Sony. Most will be listed because they are under some restriction for personal use only but can not be redistributed. Nothing stopping you from going there and getting it for yourself.
Others are just massive collections, so you can pick and choose. My interests are mostly history, so that is where the focus will tend to be.
Strategic Studies Institute, United States Army War College - Studies on current military and political problems as well as histories. These books, monographs, op ed pieces all in PDF.
Australian Defence Library - A large collection about defense problems and strategy, including 102 items of history including the entire set of histories covering Australia's role in World War II. All PDF.
New Zealand Etext Centre - A large collection of mostly .lit files on New Zealand History and culture, including biographies, literary criticism, science and natural history. Includes a number of works on New Zealand's role in World War I and II, including a work by a WWI war resister.
The NASA space program history section with PDFs on many subjects including not just history but the science behind putting astronauts in space.
Canadian National Defence-Directorate of History and Heritage. Numerous PDFs to download on Canadian Military Heritage and history in both French and English. Note, I have converted the 3 volume work Canadian Military Heritage to Sony Reader format and it is available in the LRF section. Its formatting may be more predictable than the pdf format.
U. S. Army Center for Military History - some PDFs, some html that might be the basis of a book you create covering from before the Revolutionary War to the current day. Note I have created an LRF of American Military History available in the .LRF section which may display more predictably than the PDF work.
Federation of American Scientists - HTML and PDF book on Counterintelligence in the US from the Revolutionary War to the current day in 4 volumes.