Right now, when the user opens a page or a book the app downloads all the files and, once the download finishes, then it renders the resource. If the page or book has big files like videos, then the app can take a long time to load the contents, causing a poor UX.
One possible solution is that the app only downloads the needed HTML, and the rest of files would only be downloaded when they're displayed/used like it happens in the rest of the app. The user would need to click the Download button/option to download the whole page/book.
Another solution would be to download the files in background, but render the resource as soon as the HTML is ready.