Not to mention the directory is easy to scan, rsync, backup & so on without having to worry about breaking consistency in some database. A filesystem is perfec

Not to mention the directory is easy to scan, rsync, backup & so on without having to worry about breaking consistency in some database. A filesystem is perfectly fine, if used correctly, for the average ebook reader use-case. If you have 10 million ebooks and need to run 100 complex searches per second then you need an RDBMS.