There is currently a screen rendering issue that has been reported on Adobe Digital Editions when used on Apple Mobile devices. 

The issue means that text cannot be read. This is being investigated by Adobe, however, there is currently no estimate on when this will be fully resolved. 

In the meantime the following workaround should resolve the issue: 

You will need to change the  theme settings in ADE to black background or sepia background.

To do this follow these steps: 

1	Open Adobe Digital Editions and navigate to the title in your Library.
2	Open the book that you are having difficulty reading.
3	Select the Colour setter
4	Change the background colour to Black or Sepia
5	Read the book!