Apple Watch can sync audiobooks from Apple Books.
Note: Audiobooks from other sources aren’t synced to Apple Watch.
Go to the Apple Watch app on your iPhone.
Tap My Watch, then tap Audiobooks.
Tap Add Audiobook, then select audiobooks to add them to Apple Watch.
If there’s available storage space, the entire contents of the audiobook you’re currently listening to—as well as the one listed below Want to Read—are automatically synced to your Apple Watch. 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.
Go to the Audiobooks app iPhone User Guide.