Syncing book reading position does not sync

Thanks for writing in. Sync issues are usually one of a few specific things, so let's narrow it down.

Reading position syncs via iCloud (CloudKit) along with the rest of your library. For it to work, all three devices need:

Same iCloud account signed in on Mac, iPhone, and iPad.
iCloud Drive enabled for BookShelves: System Settings (or iOS Settings) → Apple ID → iCloud → check that BookShelves is toggled on.
The same book entry on each device. This is the most common cause. If you imported the EPUB separately on each device (rather than letting it sync over from one), each device may have a different copy of the file, and reading position is tracked per copy.
Quickest test:

On the device that's furthest along in the book, open Settings inside BookShelves and check that "iCloud Sync" shows as active (not paused or signed out).
Then on a second device, pull down to refresh the library and wait 30 to 60 seconds. Open the same book and see if the position has moved.
If positions still drift after that, it would help to know:

Are all three devices on the same iCloud account?
Did you import the book once and let it sync, or did you import it separately on each device?
Roughly what percentages are showing on each device right now?

I think best closed - iCloud had issues at the time.

Things seem OK now.

Glad to hear things are working now, Mark. We've also shipped an improvement to position sync in 1.0.24 that uses device-independent progress tracking, so it should be more reliable going forward. Marking this as shipped — feel free to reopen if you run into it again.