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Camper65 - looks like this plugin hasn't been updated to python 3 and calibre 5 compatibility, see
List of PIs to be ported to Python 3
This should help if you want to have a go at doing it yourself ==>>
Attn plugin devs: porting to python 3
FWIW: the JobSpy plugin has configuration options that allow setting of some of the Add Books preference settings on a per library basis, which may obviate the need for Quick Preferences.
BR