Another vote for 'The Plague' by Camus. I actually liked it better than 'The Stranger', but that's just me. :)
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If you like literature with an undercurrent of darkness, nihilism, absurdity and despair then (aside from the obvious, like Sartre, Camus, Beckett) I'd recommend Evelyn Waugh's novels - especially Vile Bodies, Decline and Fall , The Sword Of Honour Trilogy etc. There is the bonus that they are beautufully written (he has an exquisite prose style) and, for my money, among the funniest works in the english language.
Philip Larkin's poetry is also pretty bleak and shares a similar outlook/temper with Conrad, Beckett, Camus etc. It is also very accessible and rather beautiful.
The Gormanghast trilogy by Mervyn peake is of that ilk, though it feels dated. I find most of my books on ebay.