In case this hasn't been mentioned, there's a very old edition of letters from empress Maria Theresa and emperor Joseph II to the marquise d'Herzelles, who was archduchess Maria Elisabeth's chief lady-in-waiting and afterwards governess of Joseph's daughter, archduchess Maria Theresa. It also includes a few letters from archduchess Maria Elisabeth to the marquise.
Joseph Marie Bruno Constantin, baron Kervyn de Lettenhove (ed.), Lettres inédites de Marie-Thérèse et de Joseph II (Mémoires de l'Académie royale des sciences, des lettres et des beaux-arts de Belgique, collection in -8 °, 20), Bruxelles, Hayez, 1868.