If your ebook will not download, there are a few things to try:
1. Leave the app open and running until the download completely finishes. It won't download in the background.
2. Update your iOS to the newest version, and update the App to the newest version (Click Here).
a) Log out of app by tapping the 3-line menu in top left, then tap Signout.
3. Tapping the blue "i", on the book cover, then tap Delete, then tap Download again.