I love this set too. I've played Ophélie, Salomé, Phoebé and Desdémona. I'd say they're all between Henle 4 and 7, Desdémona is the easiest and Omphale is the hardest.
Thank you! Where did you find the music for Ophelie? I can find most of them except that one.
Based on a quick search, it was recently unearthed in 1998 and isn't in public domain, so if you want it legally you'll have to pay for it.I ran into a similar issue with Godowsky's Study no. 50 a while back. Basically, while Godowsky was writing his studies on Chopin's Etudes, he suddenly had to flee his home in Austria-Hungary due to the outbreak of WW1. He left a few unfinished studies (30a, 44a, and 50) behind. 30a and 50 were recently rediscovered, but the sheet music isn't available for free, even though the rest of the studies entered public domain a long time ago.TL;DR Copyright law can be annoying at times, and this is one of those times.
Mel Bonis is in public domain and is available for free to everybody thanks to her family and to Association Mel Bonis:https://www.bruzanemediabase.com/en/mediabase/news/mel-bonis-scores-now-available-online