Gossip on Frederic the Great

When French general Jacques Antoine de Guibert went to Germany, he also visited Frederic the Great to learn more about the 'Prussian regimental drills and army manœuvres'. Later, when he went back to France he published a journal and also commented on Frederic's non-existent sense of fashion. The Friedrichradio on RBBinfo has a podcast (German) on rumours and gossip about Fritz.

Guibert, Journal d'un voyage en Allemagne (1803); Journal d'un voyage en Allemagne fait en 1773 par G.-A.-H. Guibert, ouvrage posthume publié par sa veuve et précédé d'une notice historique sur la vie de l'auteur (2 volumes, 1803)
The French ebook is available at google in two volumes!
Personally I would like to read the love letters between him and Julie de Lespinasse...