ROUSSEAU, Jean-Jacques. Écrits sur la musique - Tome 1, 2 et 3 (Œuvres - Tome 9 à 11.) Published in Paris by Deterville in 1817. In 3 volumes, In-8 format. Full tan mottled calf, smooth spines decorated with gilt irons, gilt lettering on green leather, gilt roulette framing the boards, gilt filleting on the edges, mottled edges. Contemporary binding. 417 pp, 379 pp, 504 pp, 14 folding plates (2 in vol. IX and 12 in-fine in vol. XI). Rubbing with slight leather tears, split jaw on second plate of volume 9, top headband faded, scratches on boards, corners bent. Minor scattered foxing. A collection of Jean-Jacques Rousseau's writings on music, corresponding to volumes 9 to 11 of his works, in their new 1817 edition. Volume 9 brings together several of the philosopher's writings, including a project concerning new signs for music, read at the Académie des Sciences on August 22, 1742, a dissertation on modern music, an essay on the origin of languages, a letter on French music, a letter from a symphonist and a letter on military music. Volumes X and XI present the dictionnaire de musique, which takes up, corrects and completes the articles written by Rousseau for the Encyclopédie, and provides a very useful picture of the state of theoretical music in the second half of the 18th century. It should be noted that the title and tomes have been inverted on the 2 volumes making up the dictionary. The set is illustrated with 14 fold-out plates, including several plates of scores. A fine, homogeneous set of writings reflecting a lesser-known aspect of the famous philosopher, who was also a learned musician.