Hello al-1234,
Thanks for reaching out to the Apple Support Communities! From your message it sounds like you can see the audiobook you want on your Apple Watch, but when you go to add it to your Library on the Apple Watch it's not appearing.
If this is correct, first, check to see if you have storage on your Apple Watch to add another audiobook.
"By default, the next five hours of the audiobook you’re currently listening to, as well as the one listed under Want to Read, is automatically synced to your Apple Watch if there’s enough storage space. Five hours from each audiobook you add is also downloaded to your Apple Watch when there’s available space. Audiobooks sync to Apple Watch when it’s connected to power." This information can be found in the Apple Watch User Guide section Add audiobooks to Apple Watch.
To check your storage open Settings on the Apple Watch and go to General > About > capacity and available storage. Once you know you have enough storage, be sure you have your Apple Watch connected to power when you try to sync the Audiobook to it.
All the best!