[shentar]

Oddly, I had this same idea today. So I tried it.

I used Add Books to add an mp3. It added OK (I had to use the All File filter in the add dialog to see it). The only problem was that the Author and Title fields were inverted - probably because of the directory structure it came from. I was able to add tags and a genre to it and move it between libraries. Double clicking on the entry fired up vlc to play it back.

So as far as I can see Calibre is almost there as an audiobook manager. AFAICS we would need two things (three if you don't want to copy the files into your Calibre library):

1. A filter in add-dialog to add audio file formats.
2. A way to associate multiple files as a single "book" (many audiobooks come as multiple mp3s)
3. An add-as-link option for adding files.

Item (1) should be trivial.
Item (3) would be, I think, trivial on linux but I have no idea how difficult it would be on windows (shortcuts do not really replace symbolic links).
Item (2) is the major problem. Effectively each audiobook would need to have something like a playlist associated with it. Working on the playlist - which could be autogenerated during the add-book operation or manually generated later - would apply the same operation to all files in the audiobook. This would include move/delete/tag/set genre. By default you would probably want to hide the individual part files (some of my audiobooks contain several hundred short mp3 files to cope with the problem of restarting part way through a book).

Thinking about it, rather than beat calibre into shape to do something it wasn't originally designed to do, I'd be quite happy to have another custom metadata field (like genre) which simply gave a path to an associated audio-book directory. Double clicking on the tag would open the file manager on that directory and after that I could proceed using my normal audio tools. Maybe that is something which could be added via a plug-in?