Does anyone know how to get Apple Books and Pages into Split view?

Have you followed this?

Use two Mac apps side by side in Split View - Apple Support

Let us know where it fails for you.

Open a Pages document, and then with Apple Books, open a book in it.

Now, click and hold the green traffic light on the Pages document, and select Tile Window to Left of Screen. Then just click on the open Books content and it should automatically Tile its Window to the Right of Screen. Just worked for me on macOS 11.6.5.

You follow my directions for getting Pages to tile on the left of the screen. You have the Books application open with a book displayed in it. You then just click anywhere on that open Books application and it will automatically become tiled on the right of Pages in Split view.

I tested this before I posted, so I know that it actually works.

You have to have both apps open on the same desktop for this to work. If you're seeing no available windows it's because you haven't opened the 2nd app on the same desktop. If you can't get both apps on the same desktop you probably have the apps set to open on a certain desktop. Control click on the app in the dock > Options > Assign to all Desktops or assign both apps to the same desktop.