So does Apple want the Apple ID that owns all these purchases, or the Apple ID associated with my iCloud documents? 
I’ve been waiting for things like this to happen. For years Apple allowed us to have a separate Apple ID for purchases and iCloud service — but it’s clearly not their preference.
Meanwhile there is no way to merge Apple IDs (though it may be possible to do this with customer service).
This is going to go badly…
 In this case they wanted my iCloud Drive/Documents ID, not the Apple ID for DRMd content. I don’t have any DRMd eBooks fortunately — all purchased books are from Google Play store and any DRM is stripped.
Update: Via Twitter, @dfjdejulio suggested:
Partial workaround is to have iCloud account be head of family and store account be family member. Use iCloud account, access all content.
Clever! I may do this. Need to think it over a bit.