These are the prefatory texts for volumes 31-40 of the Bach Gesellschaft Ausgabe, offering scholarly commentary on another segment of Bach's extensive oeuvre. They illuminate the critical approaches to his music.
The study of these prefaces requires an appreciation for the evolution of Bach scholarship.
Bach Gesellschaft Ausgabe Prefaces (Books 31-40) 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 31-40) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.