Johann Baptist Cramer's "60 Selected Studies" are essential works in piano pedagogy, focusing on developing technical proficiency. They cover a wide range of fundamental piano techniques suitable for intermediate players.
Consistent practice on evenness and articulation is crucial.
60 Selected Studies is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 5.
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.
60 Selected Studies is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.