[Adoby]

'/library/author/title(id)/title - author.format'

The (id) part is a numeric counter, that is increased as you use calibre, and it is used by calibre to make the folders unique and to connect folder with a specific database entry. So it is possible to have the same book, with exactly the same title and author, without them overwriting each other.

It is easy to make mistakes if you try to manually manipulate individual files and folders in the library. Typically the effect is that calibre no longer can find the files or formats. The database maintenance tools can sometimes fix that.