On The Road, by Jack Kerouac; and Fear And Loathing In Las Vegas, by Hunter S. Thompson. I read these when I was 17 as part of an extension English class at high school, and was totally blown away by them.
Gravity's Rainbow, by Thomas Pynchon. The first book I've read that made me feel like a child again, being held above a parents head to see the crowd in its entirety. Amazing.
Stranger In A Strange Land, and Starship Troopers, by Robert A. Heinlein. Both of these gave me a different perspective on the role of people within society. SIASL made me think a lot about the byplay of sex and religion, and ST inspired a lot of moral questioning about my rights as a citizen played against the problems of unflinching fascism.
All Quiet On The Western Front, by Erich Maria Remarque. Before I read this, I had the boyish perception of fighting and war being something exciting and adventurous. This book, along with the poetry of Sassoon and Owen (among others) made me realise the truth.
Dragonflight, by Anne McCaffrey. I first read it when I was six, and it was the first adult fiction (adult in the sense of not-a-Dick-and-Jane-reader) I was exposed to. Opened my eyes.
In terms of poetry, Dylan Thomas' And Death Shall Have No Dominion, e. e. cummings pity this busy monster, manunkind and she Being Brand... (XIX) gave me an appreciation for how powerful words can be, even (particularly) in small doses. Also, Ginsberg's Howl.