You mean a system like Audiobookshelf[0]? I can highly recommend this, by the way. Works more reliably than any paid service I've ever tried.

[0] https://www.audiobookshelf.org/

I'm more interested in something that would unite audiobooks and textual books.

I love to jump between listening (in my car or while walking) and reading. Right now, only Amazon Kindle + Audible provides a good experience, but it's impossible to import your own audiobooks into Audible.

Yup, this is the goal! Library management and a reader app already exist, though there's definitely work to be done, especially on the library management front.