I actually find 'Brave New World' a lot more incisive and readable than '1984', which I think is somewhat overrated.
Other than that, I'd nominate 'Alice in Wonderland'/'Through the Looking Glass.' It probably doesn't come across on AB very much (I tend to be super-serious on AB because humour doesn't translate well), but in real life I make an effort to see the ridiculous side of everything I can.
Learning to realise how absurd most people are got me out of a particularly po-faced and humourless phase in my teens and has dramatically affected the people I choose to socialise with. Reading the 'Alice' books were at least partly responsible for that - and so was 'Catch-22'.
As Lazygun mentioned in the other thread, I've also been a long-time reader of science fiction. Probably the book I most obsessed about when I was younger was Frank Herbert's 'Dune', which isn't a particularly uplifting read, but I think it might have introduced me to the idea of transcendence. Whenever I think about that book, my mind always ends up trying to imagine being something other than human in a way that I've not found in other books. The first book alone has an incredibly rich setting which puts snore-fests like 'Lord of the Rings' to shame.
Herman Hesse's 'Steppenwolf' had a significant effect on me, too. Though I only read it a couple of years ago and I can't really explain what that effect is.