The King's letters

The Library of the University of Trier offers free access to digital copies of Frederic the Great's letters. They are available in German and French.
Check out this website!

The letters were originally published in the 1840s and 50s by Johann Preuss. Momentarily I don't know, if they are available on Gutenberg or any other free archive as well... but let me know, if you find anything...

PS: Another great resource is the website for D'Alembert's letters!