On my machine running Catalina, the solution in https://apple.stackexchange.com/a/382603/449477 worked beautifully. However, this was not the case when using the same solution on my other machine running Monterey.
I attempted to follow the solutions proposed above, but the biggest barrier was working with regular expressions passed to sed
- I'm not great with regular expressions and ran out of patience. I figured I'd be a lot happier if I could get the clipboard contents (with the citation) into Python where I could then use some tools I'm a bit more comfortable with to strip the citation.
This involved creating a Shortcut as shown below:

This passes the clipboard contents to the script, which can easily read this input using sys.stdin.read()
and then manipulate as needed. The particular implementation shown below to remove the citation is not very elegant, and may break depending what I try to copy out of Books, but it works for now. The important part is that should it ever break, I'm much more confident that I can quickly fix in Python rather than going back down the regex rabbit hole. The contents of copy-from-books-without-citation.py
are below:
import sys
data = sys.stdin.read()
remove_citation = data.split('“',1)[1].rsplit('”',1)[0]
print(remove_citation)