Plexamp is no file reader. All it does is playing content which has been catalogued by Plex media server. So this means you need to make your files consumable by Plex Media Server. There is nothing that can be done about it in Plexamp.
There is no audiobook support in Plex in general.
Which means, you need to make your files appear to Plex as regular music albums.
See Audiobook wrong metadata - #2 by OttoKerner for a bit more information.
There are plenty of these. For Windows and Mac there is mp3tag for instance. It can both apply meta data to several files at once, as well as deriving metadata from folder structure and file names (and vice versa).
