If you are using Patch-1.1.0 or earlier, delete the config and regenerate it.
It is a plugin that displays books when joining the server.
Tested only on 1.16, but may work on other versions.
If you have any problems, go to Discussion or Twitter
Commands:
Code (YAML):
/joinbook: Display this help
/joinbook setbook: Set a book
/joinbook open: Open book