The book is the result of the author's many years of work in the field of Orthodox music and her research of music collections in Bulgarian and foreign libraries. It presents over 50 unknown or little-known musical manuscripts from the period between the second half of the 17th century and the beginning of the 19th century, classified by content, repertoire, typology, authors, and inscriptions. A number of unknown writers are introduced for the first time. For the first time, an entire layer of repertoire is presented, which remains unique to date. For the first time, data is presented on the notated services of the popular Bulgarian saints Ivan Rilski and Petka Tarnovska. The author's aim is to reveal the Bulgarian contribution in a pan-Balkan context and to show the comparability of Bulgarian music with the musical processes of development in other Balkan Orthodox countries.