Identical to the previous entry, this set represents Clementi's exploration of thematic development through varied textures. It is an essential example of Classical era variation technique.
The primary hurdle is the clean execution of fast-paced broken chord figurations.
Comparable difficulty and character — good candidates for what to learn next.
21 Variations on 'The Black Joke' is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 7.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 21 Variations on 'The Black Joke' lasts about 10 minutes.
21 Variations on 'The Black Joke' is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include 21 Variations on 'The Black Joke', WO 2 by Muzio Clementi, Fantaisie-Impromptu in C-sharp minor by Frédéric Chopin, Rondo capriccioso in E major by Felix Mendelssohn — see the full similar-pieces list on this page.