It's now possible to embed files, such as Pages, Numbers and Keynote documents, PDFs or Google Docs.
These will appear as an embedded link on the page, and will pop up when you tap on them in read mode.
Here's an example, with a PDF.
1. Tap the + button and choose the More tab. Tap the Files option.
2. Navigate to the file you want to embed. This could be saved in iCloud, Files, or another cloud app like Google Drive or Dropbox. Tap on it to add it to the book.
3. For some file types (e.g. PDF), you can use the Inspector to toggle whether or not a thumbnail is shown. You can also change the title of the document, and the font/color. You can also resize the embedded document using the blue handles, like you would an image.
4. Now, to view the document, switch to read mode. When you tap on the link, it will pop up and open over the page, so you can read the file. Tap Done to go back to the book.
Audio and video
There are existing ways to add audio and video to your book, including content you've created such as GarageBand tracks. But, you could also use this "embed file" method to add audio or video to your book, simply pulling in any saved audio or video files.