I appreciate all the Mobile big searches are server side, but that got me thinking, I am privileged to have too many books in my library but only use 15 or 20-70 (depending on commentary set) of my 2000+, 99% of the time I don't care about the other books on my laptop, but wouldn't want them hidden on my PC where I would still like my full library.
Given the mobile experience, at least in theory there must be now be a way of choosing what subset of books we want locally resulting both in a smaller local index and footprint.
Desktop (first search) would be those local books, plus maybe with a new additional section for online titles only results with option to download it and add to local index, or to buy if not owned.
An additional upside would be that seldom used books would always be up-to-date online, and would not require local reindexing when typos were corrected.
Just a thought..