These are the prefatory texts for volumes 21-30 of the Bach Gesellschaft Ausgabe, detailing scholarly research and editorial decisions for this set of Bach's compositions. They offer invaluable historical and musical context.
An understanding of musicological research and textual criticism is important for engaging with these prefaces.
Bach Gesellschaft Ausgabe Prefaces (Books 21-30) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
Bach Gesellschaft Ausgabe Prefaces (Books 21-30) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.