Downloads on the Apple Watch take longer than on other devices. This is because the watch uses your phone's internet connection via Bluetooth, so you may have to wait a little while.
To speed up your download, you can try turning off Bluetooth in your Apple Watch settings and then downloading your books. This way, your watch uses Wi-Fi directly, so make sure it is connected to a Wi-Fi network.
If your download gets stuck, you can always cancel it and try again. If it still doesn't work, try downloading a different book. If the problem persists, please get in touch with us through our contact page.