Hi @Sagasu2
Welcome to the Sonos Community!
With On This Device, Sonos is provided the list of music tracks by Android, so anything indexed by Android as music (usually all audio files) will show up in On This Device in the Sonos app.
You can prevent Android from scanning a folder by placing an empty file called “.nomedia” (with the period but without the quotes) in it. So, if you were to separate your audiobooks into a folder named “books”, for example, then place a file named .nomedia in that folder, you would no longer see those audiobook files in the Sonos app. Note that other Android apps will also be unaware of the contents of the folder if they rely on Android’s built-in media scans to operate, so I don’t know if this will affect your preferred audiobook-playing app too.
I hope this helps.