These are the prefatory texts for the final volumes (41-46) of the Bach Gesellschaft Ausgabe, concluding the scholarly edition of Bach's complete works. They provide essential scholarly context for the remaining compositions.
These texts are primarily of interest to scholars and advanced students of Bach's music.
Bach Gesellschaft Ausgabe Prefaces (Books 41-46) 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 41-46) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.