I've taken a squiz at this because this was annoying me too and I think there are a few UI issues to shake out here. Mainly that while you can have multiple users syncing, there is only ever one read column in the calibre db - you need to only ever allow one user to sync with the custom column or else you can have users flip flopping the read state every time you sync which would be no good. I'm thinking this can be a selection box in the Feature Configuration menu. Perhaps only enabling this one there is one user. Or only for the Admin user. Idk.

Another issue is that the column is a boolean while there are three Kobo read states - unread, reading, and read. There's also some synchronisation questions around what to do when you toggle the read state in calibre, but then sync while you're reading in the Kobo. Needs some more investigation about the exact sequencing how a sync occurs before figuring out what to do here, though I suspect it might end up being that when the Kobo indicates a book is read, we set the value on the calibre column and that is it. Tbh its about all I wanted anyway.

Anyway, I'm still working on this, but would appreciate any thoughts.

You bring up multiple potential issues:

1. Multiple Kobo users have multiple read states but there is only one Custom Column in Calibre
   --> Valid problem, but that issue is already present. Currently multiple calibre-web users sync all to one Custom Column, so that wouldn't really change. But should probably be addressed at some point in a seperate issue
2. Kobo read state is not binary, while the boolean in the Custom Column is
   --> Currently calibre-web treats the Kobo read state as binary anyway - "finished reading" and "not finished reading" - so again, there wouldn't really change much