I actually find Notes from the Underground by Dostoevsky to be the most powerful, and relevant- but I agree Camus' Myth of Sisphyus is just as important.
I found this on the book: Sisyphus is the absurd hero. He is, as much through his passions as through his torture. His scorn of the gods, his hatred of death, and his passion for life won him that unspeakable penalty in which the whole being is exerted toward accomplishing nothing. It is during that return, that pause, that Sisyphus interests me. A face that toils so close to stones is already stone itself! I see that man going back down with a heavy yet measured step toward the torment of which he will never know the end. That hour like a breathing-space which returns as surely as his suffering, that is the hour of consciousness. At each of those moments when he leaves the heights and gradually sinks toward the lairs of the gods, he is superior to his fate. He is stronger than his rock. If this myth is tragic, that is because its hero is conscious Sisyphus, proletarian of the gods, powerless and rebellious, knows the whole extent of his wretched condition: it is what he thinks of during his descent. The lucidity that was to constitute his torture at the same time crowns his victory. There is no fate that can not be surmounted by scorn.
If a fool would persist in his folly, he would become wise (William Blake).
The road of excess, leads to the palace of wisdom (William Blake).
Arguments are to be avoided; they are always vulgar and often convincing (Oscar Wilde).