Since Pocketbook supports folders and subfolder and my mistrust of library apps, I manage collections with calibre on Pocketbooks exporting it like this f.i.:
\EBOOKS\Dumas,_Alexandre\D'Artagnan_Romances,_The\ D'Artagnan_Romances,_The_3.30-Man_in_the_Iron_Mask,_The.epub
But I know of other Pocketbook users, who are relying on the ability of the pocketbook library app to properly read the correspending "Series" tag.
BTW. there is a new app from user @Reaty, which is supposed to help with collections on pocketbook:
https://www.mobileread.com/forums/sh...d.php?t=334784