Book list is empty when trying to sync Books with iPad or iPhone

Hello tomasaur,

Welcome to Apple Support Communities. to get started, please back up your devices, and install any available updates. We want to ensure all devices are running the latest version of the OS available.

Update your iPhone or iPad - Apple Support

Update macOS on Mac - Apple Support

Next, make sure you are signed in with the same Apple ID on all your devices. If so, ensure iCloud drive is toggled on in: Settings > Apple ID > iCloud.

If the issue persists, Reset network settings on each device involved.

For iPhone, and iPad:

Reset your network settings

If you're using iOS or iPadOS 15 or later, tap Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings.

If you're using iOS or iPadOS 14 or earlier, tap Settings > General > Reset > Reset Network Settings.

This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.

If your iPhone or iPad won't connect to a Wi-Fi network - Apple Support

For Mac,  forget the network and restart.

Forget a network on Mac

Follow the appropriate steps, depending on which macOS you're using.

macOS Ventura or later

To forget the network that you're currently using:

1- Choose Apple menu  > System Settings.

2- Click Wi-Fi in the sidebar.

3- Click Details next to the network you're using.

4- Click Forget This Network, then click Remove to confirm.

To forget a network that you've joined before but aren't currently using:

1- Choose Apple menu  > System Settings.

2- Click Wi-Fi in the sidebar.

3- Scroll down on the right and click Advanced to see the networks known to your Mac.

4- Click the more button  next to the network, choose Remove From List, then click Remove to confirm.

Earlier versions of macOS

1- Choose Apple menu  > System Preferences.

2- Click Network.

3- Click Wi-Fi in the sidebar.

4- Click Advanced to see the networks known to your Mac. 

5- Select the network, then click the remove button (–) beneath the list. Click Remove to confirm.

6- Click OK, then click Apply.

How to forget a Wi-Fi network on iPhone, iPad, or Mac - Apple Support

Finally, let's test by reconnecting to Wi-Fi on each device and add a new document to Books. 

We hope this information is useful. Kind regards.