Tompe: the book is not described in the article as a scientific study, and I would guess,
from this review, that it is not. This book is about is the opinions of Sven Birkets, not scientific facts.
Personally I would very much like to see statistical facts about this. This kind of opinion belongs to the "new generations are degenerate idiots" category, which has been around for centuries.
Here is an article to read about the illusions we may all have about "things going down the drain nowadays". There are, by the way, many more interesting, amusing and thought-provoking posts on this blog on similar subjects.
Just because we feel the world around us has become too fast-paced for book reading does not mean that less people in this generation are able to read a "difficult" book than in the previous generations. It may be that books that were considered "normal" read 20 years ago are found difficult by younger generations. But it's also possible that the people who read those books 20 years ago would be baffled by some of the books younger people read.
I am not knowledgeable enough about these things to form an opinion myself, but until I find some hint that documented statistical evidence backs these "studies", I will treat them with caution.