The problem of Viasheslav (and maybe of others) is not yet addressed. I can see if it is possible to disable the indexing and create a kind of 'lazy-indexing'. This would mean that only the SD-card view would work and that, when entering a folder its current content would be read and added (or replace) the memory-database-view-content. This would add some delay when showing folders.
This would also mean that features like search and the 'views' do not work.
Should this mode then be enabled automatically when there are 'too many' books on the SD-card or manually with an option in Settings?
A hybrid mode would also be possible, limiting the files that are indexed (and thus available in the non SDCard-view) and using the lazy-indexing in the SDCard-view.