Look into a bit of software called Calibre.

It can source metadata including synopsis from several locations. That can be done in batch mode. The lookup depends on it finding the isbn # inside the book. Works for the majority of ebooks. The rest will need to have the isbn set manually.

Plus it can provide a home content server for your books.

Don't know if it can manage audiobooks, never tried. There is a plugin. This post discusses the plugin.

This post lists many sources for book metadata.