Hi there!
That error can mean one of those things:
1 - You don't have EliMZ_Book, which is my core plugin. If you do not have my core plugin, RPG Maker MZ Plugin manager will warn you with a message on the bottom of the plugin manager window.
2 - You have EliMZ_Book. But it is outdated.
3 - You have EliMZ_Book, but it is below Pixel Perfect plugin. The EliMZ_Book need to be somewhere above all other Eli plugins.
4 - Some plugin parameter from EliMZ_Book is not properly configured and is triggering an error making RPG Maker kind of not recognizing the EliMZ_Book. Make sure there is no plugin parameter empty, unless the description says that it can be empty
So make sure you fix those three steps above. If the issue still happens:
5 - Send me ascreenshot of your plugin manager, with your plugin list.
6 - When the error happens, press F12 or F8. A browser window will open. Go to the CONSOLE tab, and send me a screenshot of what is in there.
Link to EliMZ_Book → Hakuen Studio Eli Book RPG Maker MZ by Hakuen Studio