I'm looking for Mac and iPad apps to replace my current iBooks (now Apple Books) workflow.
Even before starting to use iBooks, I had a folder in my Mac with eBooks (mostly PDF and ePub, a few DjVu), which I manage and categorize into subfolders myself. I don't like that iBooks forces me to make a second copy, in a "hidden" folder, of the books I want to sync with my iPad. I feel it's quite obvious why having two different libraries (my own, manually managed, and iBooks') is undesirable.
Since I download new eBooks both from my Mac and my iPad, I need a two-way sync feature: new files on the iPad should be stored on the Mac, and vice-versa.
I would like to avoid any cloud storage-based solutions if possible, but as a last resort, I may have to go with one. My rationale is that, although currently I have available space in Google Drive and OneDrive, should this situation change in the future, I don't want to be stuck paying a monthly fee to support my eBook workflow, considering I have the available space both on my Mac and my iPad to store the files.
I understand it's unlikely that I'll find a free solution. I'm willing to spend some money on this (say on the order of $50) for Mac and iPad apps to implement my new workflow.
Currently I see three paths that I could take:
Have something like Calibre on the Mac and find an iPad app that interfaces with it. I bought MapleRead SE, which can use Calibre's content server, but there's no syncing back to the Mac. I also read about Calibre Companion, but it doesn't have a built-in reader, so I have to copy books over to another app like MapleRead (wasting precious storage space on my iPad), and then I'm back again into the issue of having two different libraries, this time on my iPad: Calibre Companion's library and MapleRead's library. Also, it is unclear to me whether Calibre Companion supports two-way sync, which is a requirement for me.
Some iPad eBook reader apps, like KyBook 3 (which I've already downloaded on my iPad), use the file management features of the new iOS (i.e. they create a folder for the app in the iPad's Files app). My problem would be solved if there was a Mac app that connects to the iPad and keeps this folder in sync with my Mac's eBooks folder. Sure I could use iTunes file sharing to do this manually, but it won't scale for a large library. I need something that syncs automatically.
As a last resort, I could move my (currently local to the Mac) eBooks folder to a cloud storage folder like OneDrive or Google Drive, and then find an app that syncs with it. KyBook 3 claims to do so, although I was unable to find this option in the app (I only found mentions to iCloud drive, but the services where I have a lot of space available are OneDrive and Google Drive). Nevertheless, I'm sure I'll find it with some digging, but as already pointed out, I'd rather avoid any cloud storage services if possible.