The percentage of people reading books (other than for school) has changed less than the sample error so it may in fact have gone up rather than down. It has never been that high in history -- from the early priests that kept the others "in the dark" to the modern day, reading has always been an activity of the upper educated (school or self.) If it was popular then we would have more square feet of libraries than we would of prisons.
In today's world a book represents an investment of time and mind, a magazine can be sampled on the run.
If I read a novella in Analog it is classified as a magazine, if I read the same story as a standalone it counts as a book. Go figure.