Jean-Sigismond Cousser, born Johann Sigismund Kusser in Hungary and died John Sigismond Cousser in Ireland, began his career in Stuttgart, held various positions at several German courts before being appointed director of the famous Opera am Gänsemarkt in Hamburg, then formed his own travelling opera troupe, settled in London as an independent musician, and ended his life as ‘Chief-Composer and Music Master’ in Dublin. Praised by the musical elite of his time, he is now considered one of the overlooked composers—those who, due to their fragmented careers, do not fit into the categories established by 19th-century musicology. Throughout his life, Cousser kept a Commonplace Book, in which he took notes on music theory, instruments, and performance practices; compiled an address book, recipes and remedies—in German, English, Italian, French, Latin, Dutch and Irish Gaelic—but above all, it contains over a thousand references to works by French, English, German and Italian composers spanning an impressive range of forms, attesting to Cousser’s vast musical culture and the diversity of his interests.