I admire the simplicity of #2.
Perhaps some simple search function to match {author/title (***)} as a folder search?
It would support both syncing the entire library to device via dropbox/whatever, and saving to disk then exporting that somehow -- USB or dropboxing that for transfer -- serving as a way to add books even when wirelessly connecting is not an option for whatever reason.
Much superior to #3 I think.
Perhaps it can also integrate into cloud storage apps to search for and download those folders, which would steal the only advantage that #1 has too.
I am NOT saying import the database and stuff -- just search à la #2 in cloud apps too. Obsolescence can stuff itself,
since we are treating it just like an imported save-to-disk. Covers can be downloaded if you choose the right folder -- but you'd do that anyway, together with the books itself.
I don't know about the difficulties of supporting cloud APIs, although the idea does come to mind that you do not necessarily need to support every provider out there.
I imagine just dropbox would be enough, really.
Then again, I can just see people saying "you support Dropbox (and even every other popular provider out there), why don't you support provider $weird also!!!" Maybe it's best not to support any no matter what.
