These are the prefatory texts for volumes 11-20 of the Bach Gesellschaft Ausgabe, continuing the scholarly discussion of Bach's works. They provide critical apparatus and historical background for a significant portion of his output.
Familiarity with scholarly editions and their historical context is key to understanding these texts.
Bach Gesellschaft Ausgabe Prefaces (Books 11-20) 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 11-20) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.