"has the proportion of "book readers" in the NA population grown or remained static, or, as many fear, actually diminished in the last 25 years?"

Interesting coincidence:  Robert Runte, who recently retired from teaching student teachers at the University of Lethbridge, recently gave a talk about the changes he has seen over the past 25 years in the young people who are going to be English teachers.  In the beginning, they read books, and some of them wanted to write books.  By the time the quarter-century was over, he had students who found it difficult to read magazine articles, and none of them wrote anything more than a tweet or text message.

Worth a listen:  http://whenwordscollide.libsyn.com/2016-004-goh-robert-runte