I set my NSExtensionActivationRule to TRUEPREDICATE, and it appears as a Share option in Safari (and even within iBooks if I try and share text from within a Note). But if I try and share text from an epub it doesn't appear.
Facebook is the only app that appears for me (Twitter may also show up) as a third-party app extension within Books. Is this something special that they're doing, or an Apple decision to only allow certain Share Extensions in iBooks? If so, is there any workaround for it for local app deployment?
This was answered by the Apple Support forum here.
There's no supported way for you to get text from a book in Apple Books with the APIs currently available...There are a predefined set of apps allowed to receive data from Books. This is not achievable with our presently shipped configurations.
Filed Radar FB15013575 as a feedback/feature request.