StoreKit - Non Consumable download

Hi
I'am having trouble figuring out how to organize my app logic on this problem:

the user can IAP a non-consumable content (let's say a book in .xdatamodeld format) and download it when bought. -> SKDownload ?
he also can read the book and add comments to pages - on his own database (not the book's one). -> two .xdatamodeld ?
this book can be updated if needed from the developper's side.

I was thinking of storing my contents using inApp Purchase Content. And allowing the user to download it to the dedicated Purchases.xdatamodeld? on the his cloud/or local when bought.

But how to merge all the bought content to one .xdatamodeld and how to keep it up to date ?

Thank you for your feedbacks.

Tim