The emphasis here (mine) is a widely known quote from Kafka, which I am excerpting with slightly more context then it usually has:
I think we ought to read only the kind of books that wound and stab us. If the book we’re reading doesn’t wake us up with a blow on the head, what are we reading it for? So that it will make us happy, as you write? Good Lord, we would be happy precisely if we had no books, and the kind of books that make us happy are the kind we could write ourselves if we had to. But we need the books that affect us like a disaster, that grieve us deeply, like the death of someone we loved more than ourselves, like being banished into forests far from everyone, like a suicide. A book must be the axe for the frozen sea inside us. That is my belief.
In a former life, I focused on the wrong part of this sentiment, the whole Art As An Endurance Test philosophy. That was the part of my life when I thought of stuff like Pasolini's Salò as the apex of what we should be aiming for. (I'm starting to realize that Pasolini had a breadth of talent that singling out Salò as his singular achievement does terrible injustice to.)
I've long felt it was a bad idea to advocate for books in the Kafka way, if only because most of us hear "like a disaster ... like a suicide" not as a recommendation, but as an affront. I suspect that's also because of the way most of us think about being happy is not the same as what he has in mind either. Most of us want to be happy without knowing what that really means, because we think happiness means bliss or ecstasy, states that cannot be sustained effectively. Some of us try to sustain it anyway (like that fellow we all know who drinks even when there's no party going on), with predictably wretched results.
To my mind, happiness is a lot like Václav Havel's notion of hope. It's not the idea that everything will be okay, but that we will somehow be able to make sense of all that has happened. Likewise, happiness to me isn't about feeling good all the time, but about being able to bounce back from both heights and depths, to always be able to find a good ground to return to. If you have that, then books that attack you, that break up your frozen sea, are going to be palatable, even desirable, because reading them won't feel like a wound you inflict on yourself. They will feel liberating, revelatory. The frozen sea inside us is not really us, but ice from the cold of the world outside.