Easiest way is to use the second "Add books" option as per the screenshot.
Select the Library folder, and it will search all folders inside and import each folder as the right book, with all the metadata inside.
You can use the command-line version as well:
Code:
calibredb add --recurse --one-book-per-directory /path/to/library/
See the command-line documentation for calibredb add here:
http://manual.calibre-ebook.com/gene...#calibredb-add
General command-line documentation for calibre:
http://manual.calibre-ebook.com/gene...cli-index.html