This is not a restriction, but rather an enhancement of my Sunday observance. The books can be fiction or non-fiction, physical books or books read online. However, they are not to be too scholarly, and a book that I read on Sunday must be one that I would enjoy reading on Monday.
After all, the idea is to enhance my Sunday observance, not to make it dull.
Of course, there are certain books I read every day, Sundays included. For example: The Bible, "My Imitation of Christ", and Karen Edmisten's lovely book, "Through The Year With Mary: 365 Reflections".
I have found that I look forward to that weekly change of pace in my reading. I have also found that I enjoy the weekday books all the more for having put them aside for 24 hours.