I am interested in books by academics that examine the following. How to Read a Book: The Classic Guide to Intelligent Reading is the only relevant one that I found.Suppose a reader to be deciding which books to read about a topic. Why do some readers:judge some books easier to read than others? comprehend some books faster and retain more than others?What do authors (not) do that affect 1 and 2 above?   
What makes a book easier or harder to read?
reading