sync iPad and MacBook without losing data

I want to sync the contents of the Apple Books app between an iPad and my MacBook. Both currently have content. In the past, there was an incident in which synching two devices together (or maybe it was synching a device with iCould) for the first time resulted in all the content on one device being replaced with the content from the other. What I want is to merge the content from both. How can I do this, and ensure that I don't lose any data?

I am trying to synch this "the old-fashioned way" because I don't have enough space left on iCloud, and don't especially want to pay more for another plan.