Georges Lamothe's Billet doux is a characteristic nocturne reflecting the sentimental style favored in French salons of the late 19th century. It emphasizes expressive melodic lines and harmonic coloring.
Pay close attention to the voicing of the melody, especially when it moves into the inner voices.
Comparable difficulty and character — good candidates for what to learn next.
Billet doux (Op.177) is rated Henle level 3 of 9 — late beginner repertoire, roughly ABRSM grades 4–5. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 3 — meaning you can already play level 2–3 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 Billet doux (Op.177) lasts about 4 minutes.
Billet doux (Op.177) 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 Brise des nuits by Georges Lamothe, Bella by Georges Lamothe, Barque jolie by Georges Lamothe — see the full similar-pieces list on this page.