The first field (index) in .calendar.book files had to be changed from a sequential index to a unique ID in order to support concurrent access of calendars; one unfortunate consequence is that .calendar.book files won't be backwards-compatible with OWM 2.10 or earlier.
Other fields added to the .calendar.book files: private flag, and creator (mainly to determine which entries in your calendar were created by other users so they can edit them)
Currently, when calendars are shared, the .calendar.book files become world-readable, so if other users have access to your files via a shell (or maybe WebDisk? this didn't occur to me), they can read even your private calendar entries. One possible solution is to have an additional .calendar.book.private file for private records.