[Adoby]

Most e-readers handle "folder view". So you can put books in folders and group them on series or whatever way you like. And in the reader you can browse the folders and discover and open books.

I don't know of any Android e-reader that doesn't handle "folder view", including the free ones.

I use Librera Pro. It also has the option of hiding or showing other ways (search/series/author/title/added and so on) of viewing your books. So you can have the tab for folder view very prominent and obvious. I only have the "Folders" tab and the "Preferences" tab visible.

If you browse your library some other way than by folder then you are depending on correct and normalized metadata embedded into the book files. The same name of the series, and numbering, in all books in the same series. The same spelling and initials of authors. And so on. That takes a lot of manual work in calibre to achieve. Grouping in folders is much faster.

Librera can also be used for audio books, but I haven't tried that myself.

And I copy/sync whole folder trees, saved to disk by calibre, over the network from my PC to my reader. So you may need some app for that as well.