Hässler's early works often blend the improvisatory nature of the fantasy with the structural requirements of the sonata. It is an excellent example of late 18th-century German keyboard aesthetics.
At Henle 4 this is an intermediate piece that expects secure technique and real attention to phrasing. The main demands: precise articulation — the difference between crisp and blurred is the piece; Alberti-bass accompaniment kept quiet and even under the melody; and fine finger independence within the hand.
Beyond that, expect ornaments that need to sound effortless, not squeezed in.
At roughly 10 minutes, stamina and memory become part of the challenge — plan practice in sections.
The challenge is to seamlessly transition between the improvisatory fantasy sections and the more structured sonata themes.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Where this piece's skills lead: slightly harder works making similar technical demands.
Lesser-known works by the same hand, at a comparable level.
Fantaisie et Sonate (Op.17) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 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 Fantaisie et Sonate (Op.17) lasts about 10 minutes.
Fantaisie et Sonate (Op.17) 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 Caprice et Sonate by Johann Wilhelm Hässler, Caprice et Sonate by Johann Wilhelm Hässler, Fantaisie et Sonate by Johann Wilhelm Hässler — see the full similar-pieces list on this page.