If you've already read the masters of literary misery mentioned in yesterday's Independent article and don't find them bleak enough, then here are two others that truly corner the market on existential despair:
Anna Mitgutsch's Jakob. Out of print but still readily available used, it's the story of a woman who has an autistic son back when the mother was relentlessly blamed and scorned for causing the condition.
Barbara Gowdy's The White Bone. Elephants. They suffer. They die. They don't deserve it! Even the cover on this one cuts me like a knife.