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:

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.