Oesten's Amusements are generally charming and accessible pieces designed for enjoyment and technical development. Op. 39 likely features a collection of pleasing short pieces with lyrical melodies.
Cultivating a beautiful singing tone and expressive phrasing is key to these lyrical miniatures.
9 Amusements (Op.39) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 6.
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 9 Amusements (Op.39) lasts about 15 minutes.
9 Amusements (Op.39) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.