How do you find them on Spotify at the moment? Is it just by searching "audiobook" and then shifting through all the random stuff that brings up, some of which are audiobooks?
EDIT: I've just found https://community.spotify.com/t5/Content-Questions/Audiobook... which says
> In Browse > GENRES& MOODS > WORD you can now find these options.
Which should be available through their API using https://developer.spotify.com/documentation/web-api/referenc...
It's not immediately obvious how I get to the Browse section through the awful interface at https://play.spotify.com but you can jump straight to https://open.spotify.com/genre/word-page