This is a contemporary character piece that functions like a modern short nocturne or lyrical reflection. It relies on minimalist textures and expressive, slow-moving melodic lines.
Achieving an intimate, 'breathless' quality through the use of sensitive rubato is the main performance challenge.
Comparable difficulty and character — good candidates for what to learn next.
Bill's Asthma 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.
Bill's Asthma 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 Body and Soul by Johnny Green, Fantasy in D minor by Wolfgang Amadeus Mozart, Prelude in E minor, No. 4 by Frédéric Chopin — see the full similar-pieces list on this page.