Repurpose your blog posts, newsletter articles, or PDFs into an eBook or dynamic flipbook.
You can import content into the draft of the eBook. It doesn't all have to be done at the beginning.
Unless you are on the premium plan or higher, you cannot create your own custom template to use. You would have to customize each eBook you create.
In case you missed this previous article: How to Make a 3D eBook cover image like this one...