If a user uses several devices to read same book (f.i., a smartphone to read when travelling, and a table to read at home), it would be very handy to synchonize reading position across these devices.
The easiest way to implement it:
1. When the app is deactivated/closed (i.e. reading is paused), it should save the position (let it be a special file in the same directory where book is stored - see explanations below).
2. When the app is activated/started (i.e. reading is resumed), it should load the position.
How to synchronize: if the position is saved next to book file, the user may utilize third-party (i.e. external) cloud services (I would vote for BitTorrent Sync) to synchronize both own library and reading positions.
I suggest to keep the reading position for each book in a separate file with name similar to book file, so it would be easy to move them together between directories and track changes.
Just a thought... but my view on how this would work ideally:
For the cloud services, I believe the most useful would be the google drive associated with the account being used on the phone (Android), like the app called DataSync does. It would be wonderful because you wouldn't force anyone with an android device to open an account on another cloud service.
For the windows pc version, it would just be a matter to configure the login on the settings and use google login services to establish a secure token, limited to acces only coolreader data on the google drive, that could be stored on the machine with mimimal risks.
Anyway, whatever the cloud service(s) used, this feature would be a dream come true, on the best e-reader available (coolreader), making it unbeatable. Contratulations to the dev for it.