[innes]

Changes in these traits ... were larger in magnitude than changes typically observed in healthy adults over decades of life experiences, the scientists say.

Clearly most people don't read good books, or scientists would have seen evidence of their radical personality-changing effect.

[randallsquared]

I'm not sure if you're being sarcastic, but I would say it is certainly true that most people don't read good books. Most people don't read books, in general. Of those who do, most read for entertainment (like me), rather than seeking out great books to have read.

[innes]

No sarcasm. I have read good books (I think), but am pretty sure that they didn't modify my personality. Normally they result in a couple of days of thinking "wow, that was a good book".

[randallsquared]

I think that a "good book" in the sense used above is a book where you spend the next coupla days thinking about the new ideas from the book, or the new vantage on a subject you thought you understood, or examining your life in light of what you just read.