Hi Russell Menko,
I understand you wish to share iBooks purchases with your adult daughter. Although Family Sharing is a great way to share purchases, it can not be limited to just one section of the iTunes Store.
So, the only way you can share you iBook purchases with your daughter is to give her your Apple ID and password. Here again, she would have access to all of your purchased content, which may not be what you want.
The other option is to periodically gift a favorite title from the iBook Store to her. I'm sure she'd appreciate your thoughtfulness.
Send iTunes Gifts - Apple Support
https://support.apple.com/en-us/HT201783
- Sign in to the iTunes Store with your Apple ID.
- Find the item in the App Store or iTunes Store that you wish to gift.
- Click the arrow to the right of the price and select Gift This.
- Type the email address for the person you want to send the iTunes Gift to.
- If you want to, add a message.
- Pick the day you want to send the gift and click Next.
- Select a theme for your gift and click Next.
- Confirm your gift purchase and click Buy Gift.
Have a great day!
- Judy