And anything Ligotti has written, existential despair is his only theme. I think Teattro Grottesco, his most available work, also contains his best. His argument can be boiled down to the perspective that life isn´t worth living; life is a negative state of being, whereas non-existence is a neutral state of being, so it wins out. Anyone who claims otherwise is merely a victim and part of the conspiracy (of course, anyone who agrees with him is as well). Regarded by many as the preeminent living writer of horror/Weird fiction. He often cites this essay by philosopher Peter Wessel Zapffe as an inspiration:

http://scratchpad.wikia.com/wiki/The_Last_Messiah

I´ll include a link to one of his stories at the bottom.

A more optimistic approach is put forth by Hal Duncan on his blog:

Nihilism -- comes in two flavours, "Why bother?" and "Why the fuck not?" The former is failed nihilism, a defeatist ennui born of existential angst and nausea, negative valuations of fear and disgust in the face of a cosmos which is merely meaningless and which therefore calls for a neutral valuation. It is, in truth, fatalism rather than nihilism, validating its surrender to inaction by confusing the absence of any truly worthy cause with the absence of any truly worthy effect, projecting hostility and futility where there is only indifference and uncertainty. For the true nihilist it doesn't matter that it doesn't matter. So fucking what?

This is explored in his two-part novel The Book of All Hours, particularly the first volume, Vellum. His entertaining pulp novella Escape From Hell can be read as a summary of the same theme.