Adolfo Fumagalli was a virtuoso who specialized in composing music for the left hand to showcase unprecedented levels of agility and power. This work represents the extreme end of 19th-century keyboard virtuosity, requiring the left hand to cover the range and complexity typically shared by two hands.
The work requires extreme endurance and the ability to manage large leaps and complex voicing solely with the left hand.
Comparable difficulty and character — good candidates for what to learn next.
2 Concert Studies for the Left Hand Alone (Op.18) is rated Henle level 9 of 9 — virtuoso repertoire, roughly concert / virtuoso level.
You should be comfortable at Henle level 9 — meaning you can already play level 8–9 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 2 Concert Studies for the Left Hand Alone (Op.18) lasts about 8 minutes.
2 Concert Studies for the Left Hand Alone (Op.18) 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 2 Concert Studies for the Left Hand Alone by Adolfo Fumagalli, Pictures at an Exhibition by Modest Mussorgsky, Ballade No.2 by Franz Liszt — see the full similar-pieces list on this page.