Hi
If I can jump in again, this is similar to what I said earlier that loading chapters of the book seems to help.
But additionally, it really does help to leave the watch on charge overnight to let it complete.
It might say it's downloaded and it looks like it should be working but in reality, it's best to leave it another few hours for it to finalise. That's why chapters/sections or shorter books seem to work better: it just takes a lot longer on the bigger titles.
If you assume it's going to take about 5 hours or so to download a book and get it working, you're probably nearer right than not but it may well say it's downloaded in 45 minutes. You do really need to leave it on charge too otherwise it seems to pause it.
Now playing works but it's playing it off your phone of course not off the watch so you can't leave the phone behind.
And similarly, if you want to change to another title, you have to go back to the phone.
It's a pain.