Casella’s work is characterized by a dry, neo-classical wit and an emphasis on contrapuntal complexity. This piece is distinctive for its acerbic harmonic language and rigorous, almost academic, development of its core motif.
Controlling the independence of multiple contrapuntal lines while maintaining a cold, precise articulation is the primary task.
Comparable difficulty and character — good candidates for what to learn next.
21+26 is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 8.
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.
A typical performance of 21+26 lasts about 5 minutes.
21+26 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 Berceuse Triste by Alfredo Casella, Berceuse Triste by Alfredo Casella, 13 Variations on 'Es war einmal ein alter Mann' by Ludwig van Beethoven — see the full similar-pieces list on this page.