How does syncing work on Apple Books?

If I don't have any books on my mac and I sync with the ipad, where the books are. Will the books on my ipad get deleted or stay? I have an aunt who has an old ipad, now she got a macbook and would like to transfer her books to it, but she is afraid that if she syncs, she will lose her books. Given the age of the ipad, using Icloud is a bit unrealistic, so I'd like to solve this by dragging it through the cable and I'm worried about the books not being completely deleted during the sync, since the imac is new and doesn't have any books in it yet. Any suggestions for a solution to my problem would be much appreciated. Thank you.

[Re-Titled by Moderator]