Your comments

I had already explored the possibilty to add an epub reader in Ubooquity, but at the time none of the libraries I had tried (Monocle and Readk.it were working correctly).
I did not know epub.js. I'll test it and consider using it in the future. Thanks for the info !
I'll add it to my todo list.
(this sorting criterion is already available for comics ("Sort by path") but not for ebooks indeed)