How to share iBook collections with family members (no, not purchases or purchased books)

Family Sharing is designed for Apple purchases and subscriptions to Apple things (Download family members' purchases on iPhone). I think if you want to share a non-purchased item then you have to do that at a more basic level such as copying the book files from your computer library to the personal user libraries to which other people sync their phones. If you share a computer then they have to have a copy of the file in their account. They would then sync the items to their devices. At least that's how you have to do it with music files you have from non-Apple sources.

Sync books between your Mac and iPhone or iPad - Apple Support

Apple doesn't explain why but one line of reasoning is it might ultimately have to do with many media items being under copyright. If you are buying it from Apple then they know the initial distribution (namely you the purchaser) and that you sharing it is allowed under the Apple system. If you have something that originates outside the Apple media market system then you should easily be able to get a copy from the original source onto whatever device is used to then sync to your i-whatevers. Additionally, the company doesn't really profit by going the extra step supporting distribution of media that does nor originate with them. They don't stop you but you have to do it certain ways.