[Plugin: WPBook] Custom field still being applied after uninstalling…

Not sure what you mean by “applied” – are you still seeing the Meta box in the post edit screen, or are you just seeing the custom fields in the custom fields box?

Do you mean you want WPBook to delete all the post_meta fields it created on uninstall?

They aren’t affecting anything if you’ve uninstalled, you can certainly delete them directly in the post edit screen – but I can also look into having WPBook do so on uninstall.