Proper audiobook support is being considered (Support for audiobooks - #1070 by McWanke) but it’s not on the roadmap yet AFAIK, so considering a separate app is probably jumping the gun a bit.
Plexamp already has some audiobook-focused features via Settings > Experience > Player > ‘Use Long-form Controls’ and ‘Music Playback Speed,’ so my guess is that if Plex ever does implement a separate Audiobook library type, it would be integrated into Plexamp first opposed to a separate app.
As long as your audiobooks are in a separate library, Plexamp shouldn’t show them if it’s not your selected music source (Settings > Source).
