I am inclined to agree with the sentiment of your post as I also enjoy reading. At the same time however, I'm not sure I could provide a summary of the themes from The Grapes of Wrath as the author suggests even though I've read that book in the last 10 years.
What exactly do you mean by being an attentive reader and how do you go about training this skill? I read a lot and feel like I understand most of what I'm reading but it doesn't seem like most of it sticks for more than a year or two (if that).
"Curiously enough, one cannot read a book; one can only reread it. A good reader, a major reader, and active and creative reader is a rereader."
Exploring many different books is fine, but I think what really pays of is sticking with the books that are particularly meaningful to you and exploring them over and over again, taking notes, finding people to discuss them with, mulling them over, and so on.
I also do read quite a lot but on occassion I will always go back to the 5-7 books that are special to me, as a result I know a lot of them by heart even if I haven't picked them up in years.
What exactly do you mean by being an attentive reader and how do you go about training this skill? I read a lot and feel like I understand most of what I'm reading but it doesn't seem like most of it sticks for more than a year or two (if that).