I think Quirrell would believe that seeing one's best friend imprisoned/subjugated/otherwise punished unfairly would have more of an effect than a book, even on Harry. I think he would be right, too.

It turns out, after a few minutes' contemplation, there was a previous discussion between Quirrell and Harry lurking in the background of this thought. From Chapter 60, The Stanford Prison Experiment, Pt 10: (emphasis added)

There was a pause at this. Then the boy said, "Professor, I have to ask, when you see something all dark and gloomy, doesn't it ever occur to you to try and improve it somehow? Like, yes, something goes terribly wrong in people's heads that makes them think it's great to torture criminals, but that doesn't mean they're truly evil inside; and maybe if you taught them the right things, showed them what they were doing wrong, you could change -"

Professor Quirrell laughed, then, and not with the emptiness of before. "Ah, Mr. Potter, sometimes I do forget how very young you are. Sooner you could change the color of the sky." Another chuckle, this one colder. "And the reason it is easy for you to forgive such fools and think well of them, Mr. Potter, is that you yourself have not been sorely hurt. You will think less fondly of commonplace idiots after the first time their folly costs you something dear. Such as a hundred Galleons from your own pocket, perhaps, rather than the agonizing deaths of a hundred strangers."

Harry didn't need a friend in Azkaban to be horrified at it, but visiting it had much more of an impact on him than just hearing about it.