At first let me say
for that plugin, I love it.
Nonetheless there are two suggestions I have to make after some years of successful use:
1. Can you change the type of the custom field from text to a Y/N field? I think the three states of Yes/No/undefined are a better selection.
2. As I use sigil as my e-book editor: can you replace the hex-code
with another &x....; sequence so I can edit a book in sigil too without a lot of spelling marks.
I think that would make this fabulous plugin a bit (for me a lot) better
TIA for considering these suggestions