Then use Calibre to define bookshelves. It is much more efficient than doing so on the reader itself, and may even be more efficient than organizing files via the filesystem (since Calibre allows you to do filtering via various fields). Organizing via the various shelf implementations is more powerful than organizing via a filesystem anyhow. Filesystems force you to place books in one folder, and everything must be organized hierarchically. Shelves are nothing more than tags, so you can stick things into multiple categories.