1.) The version of the Resource.
5.0.6-RELEASE-b395

2.) Version of the Spigot / Craftbukkit / Cauldron / ETC.
git-Paper-201 (MC: 1.15.2) (Implementing API version 1.15.2-R0.1-SNAPSHOT)

3.) What is the problem that you are having?
Books are broken even with normal text and display the following image after the latest update: And yes I have cleared my inventory and rejoined.

4.) Post all your YAML / Configuration files, please include a Pastebin.com link.
Config.yml - https://pastebin.com/25FJLVEm
Items.yml - https://pastebin.com/e3y7W9bF

See; https://github.com/RockinChaos/ItemJoin/wiki/Recent-Changes

You are using the old method for defining JSON events. These have changed to support new events in the latest update. The link above states this at the beginning and points to the bottom of the page for proper formatting.

Here is the proper formatting (now);

• JSON Books have been re-coded and reformatted.
  • Old JSON Books prior to this update will BREAK unless you add the dynamic itemflag to the item.
  • New JSON formatting is <TYPE:VALUE>
  • Current types are open_url, run_command, show_text, and text.
  • Replace value with the value of your intention such as a text to be shown or a command to be executed.
  • Example; <text:I am a clickable hover text component><show_text:Visit the website! \nAnyone is welcome.><open_url:https://www.craftationgaming.com>

Let me know if you need any help and if this resolved your issue.

EDIT: I also noticed you have both the old and new formatting in your JSON book.

This is why it is breaking;

      - '<hover type="run_command" value="/server tempsurvival">"&a&nClick to connect!"</hover>'
      - '&0format <<text:TYPE><show_text:Replace TYPE with the component type. \nExample: run_command, show_text, open_url.>:<text:VALUE><show_text:Replace VALUE with the text, url or command.>>.'

Correct formatting;

      - <text:&b@RockinChaos#6746><open_url:https://discord.gg/D5FnJ7C><show_text:Click to visit the discord.>

Confirmed as working after removing the "why it is breaking" line from the book.

Here is a fixed version of what you were trying to do for the tempsurvival.

<text:&a&nClick to connect!><run_command:/server tempsurvival><show_text:Click to connect to the server!>

Mkay. Thanks.