To share a group of files (series, book series, music album), you can take it out there as torrent release on 1337x. It is public torrent site, but to upload you need to apply for uploader status.

There are private torrent trackers that you can get on via interview. Redacted is a musical private tracker. My Anonymouse is mainly about ebooks and audiobooks. Releases on private trackers available only for its members.

To share a digitized book, you can upload it to:

Library Genesis - https://library.bz/main/upload/
genesis
upload

Z-Library - https://singlelogin.re/book-add

To share your entire library use P2P networks:

Soulseek - http://www.slsknet.org/news/node/1

Direct Connect - https://www.apexdc.net/download/