It means that some variable has the state 'undefined' and cannot be used as an object, which it is in this case. There should really be tests for undefined and null states, I think. Though, given the way that plugins work in MV, I can see how an individual plugin file might end up being huge if you tried to handle everything. I just think better error handling is a good thing.
This was also a problem at times in RGSS scripts, especially when using original functions that have either been overwritten or aliased. Mixing scripts by different authors will lead to a higher risk of this kind of issue.
Yanfly is aware of the incompatibility and has said he might work on his own version of the Item Book script that will work fully.
I can't check the params at the moment, but if there is a setting regarding independent items then turn that off. That should allow them to work together. Not ideal, but something to try.
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