[petrjahoda]

Hi! OK, here's how sync works.
There are two distinct things:
- Book files
- The data for those book files

Sync only synchronizes the data, not the books themselves. It uploads new data to your cloud profile and downloads any updates.
Here's what that means in practice:

1. Say you have an iPhone with a library of 100 books, stored in an iCloud folder called BOOKS.
2. You use it for 6 months, and justRead uploads your reading data for those books to your Apple cloud profile (to put it simply).
3. After 6 months, you buy a new device, an iPad. You download justRead and point it to the same library, the same iCloud folder named BOOKS.
4. Because these are the same files, the same books, justRead will download all the data for them from your Apple cloud profile: reading progress, bookmarks, highlights, and stats.

Even if you only use one device, sync is still useful as a backup of your reading data. If you don't want a backup, you can simply disable it.