Add Apple's iCloud support, so books can be read on a few Macs e.g.
to continue read on another Mac there I left on the first one.
I've only seen a single non-Apple app add iCloud support so far (PDFpen from Smile) so I'm not sure what's involved with it, but it is likely the only thing we'll be able to do unless Apple relaxes the sandboxing restrictions to allow use of something like Dropbox. Since we're already storing the last-read position, if we can do syncing between Macs, I'm sure we can sync that position too.
So that means that book file has to be always copied between Macs else it's kept e.g. In a dropbox folder.
My suggestion here for keeping track of current-page status would be some kind of simple metadata attached to the book files. Preferably, this would be something that could be easily parsed by any other reader software that wanted to add support (like an extra OPF metadata entry that the software updates every so often).
That way, it would also continue working if a non-iCloud synchronization scheme were used.