You may be receiving a blank screen on your Apple device if the link is not opening directly in the Safari Browser. Tapping the link in an email may open the link in an internet window but not necessarily the Safari Browser.
If you see a blank screen:
1. Tap the Safari icon on the screen as shown below.
This will re-open the access code in a Safari browser window. You may have to re-login but the access code should be pre-filled.
2. Go to the Downloads tab.
3. Scroll down to the ebook.
4. Click on the EPUB and then open in iBooks.
5. Alternatively, you can click the square with the up arrow and select the option in the menu for "Copy to iBooks."