Can't add audiobooks to watch from iBooks

I am just trying to add an audiobook that is in my iBooks library on my iPhone to my apple Watch version 4. I followed the instructions here:

Add audiobooks to Apple Watch - Apple Support

But it says to click "Add Audiobook", and as you can see below, that is not an option in the Watch app!

So I am stuck. I am able to add audiobooks from Audible, but not from Apple's own iBooks app. That seems ridiculous. Does anyone know how to accomplish this seemingly simple task?

Thanks for your help!