I don't know the book, so maybe I'm totally off here... IF by chance there's any of that, "people only use 10% of their brain" stuff that's been repeated over the years both out of ignorance and for personal gain (think self-promoting gurus), you can rest assured that it is false. For a quick and amusing read on the topic... http://www.snopes.com/science/stats/10percent.asp
It can still be fun of course when a book gets you thinking about things, just don't beat yourself up over a myth, you know? (If in fact that is what he's using as one of his premises. Maybe he's not, in which case you can disregard(!) )
I do know what you mean about not feeling as sharp as in the past, because I don't either. But then again, our brains have been through a lot. Even so, many BPs are still smarter than the average bear. It sure doesn't feel that way when we're depressed, but then our thoughts are lying to us anyway, you know?
Have fun with the book, by all means. I know what you mean about meatier reading at such a time. I tend to go for comedies and ones that can (hopefully) transport me a bit out of my own mind for awhile, you know?
Hope you are feeling better soon, WendyAussie!