Lev Shestov
I came across this website which collects the writings of Lev Shestov, a Russian religious-existentialist philosopher who also wrote books on Kierkegaard, Nietzsche and Dostoevsky. The basic motif of his thinking seems to be that accepting 2 plus 2 equals 4 is accepting a spiritual death. Well, almost! The website also contains lots of articles and essays written about him. This review by Emmanuel Levinas is a good short introduction and contains this nice summary of existentialist philosophy.
"In a world clarified and explained by reason, only the general counts: my destiny is nothing important, my pain is nothing exceptional, my despair is nothing unique; if I carry a sadness or a shame in the depth of my soul, that does not trouble the universal order. My speculation assigns to these things a place in the whole, and my only wisdom can only consist of my submitting to its laws. But before speculating, I exist. My existence goes on precisely in this pain, in this despair. Far from arranging themselves in a whole that would embrace them, that are all mine. They have their history, their truth, their weight, their own exigencies. I can drive them back, but I can never fully suppress them. Their voice tears my being in spite of my submission to universal necessity. My speculation, itself, is it wholly independent of them? Can it be legitimately abstracted from the human condition, for its destiny, for its death? Whatever the response that one gives to these questions, it is important to pose them, it is important to respect the internal meaning of the events that constitute our existence, before interpreting them through the universal order constructed by reason. This is the task of existential philosophy."
There is also a very good article by Czeslaw Milosz:
"The "I" has to recognize that it is confronted with a world that follows its own laws, a world whose name is Necessity. This, according to Shestov, is precisely what lies at the foundations of traditional philosophy—first Greek, then every philosophy faithful to the Greeks. Only the necessary, the general, and the always valid will merit investigation and reflection. The contingent, the particular, and the momentary are spoilers of unity—a teaching that dates back to Anaximander. Later Greek thinkers exalted the all-embracing Oneness and represented individual existence as a crack in the perfectly smooth surface of the One, a flaw for which the individual had to pay with his death. From a Shestovian perspective, Greek science and morality both follow the same path. The sum of the angles in a triangle equals two right angles; the general, eternal truth reigns high above breeding and dying mortals just as eternal good does not change whether or not there is a living man to aspire to it."