A circular import was what my google searches suggested as well. The bit that had me was I did the same thing in a user plugin and saw the error when running from source but didn't get an error when running from the install. I'll put it down to a difference in the debug vs production environment.
Using the user plugin version, I have done a performance test. For 933 epubs, it took an extra 7 seconds when the import was just before use. As far as I'm concerned, that's negligible.
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