Hello maarten_d,
Thanks for reaching out to the Apple Support Communities. I understand that you're looking for some assistance in adding audiobooks to your Apple Watch.
The Audiobooks app is available with watchOS 6. watchOS 6 is available on Apple Watch Series 1 and later. To upgrade to watchOS 6 you'll also need an iPhone 6s or later running the latest iOS 13 version. Also, check to see that the Books app is on your iPhone.
Once you're sure you have those items, check to see which watchOS version your Apple Watch is running, and if there are any updates with these steps: Update your Apple Watch.
If your iPhone and Apple Watch are already up-to-date, or you're still not seeing the Audiobooks app, follow these steps to restart both devices:
1 - Power off Apple Watch
2 - Power off iPhone
3 - Power on iPhone
4 - Power on Apple Watch
Be sure you've waited for each step to complete before proceeding to the next one. If you haven’t restarted the devices before, here’s how:
How to restart your Apple Watch
Restart your iPhone, iPad, or iPod touch
Let me know if this is helpful.
Cheers!