This one is more of a children's novel, but I read it as a kid and it always stuck with me.
A Wrinkle in Time by Madeleine L'Engle
Much Ado About Nothing - Shakespeare
The Prince - Machiavelli
The talented Mr. Ripley -Patricia H.
Lolita – Vladimir Nabokov
It's funny that Orwell's 1984 was mentioned in the post before this. I was going to say that Orwell's Animal Farm is one of the best books I've ever read. It's also easier to read.
Alice's Adventures in Wonderland -- Lewis Carroll
Nineteen Eighty-Four -- George Orwell
Three Men in a Boat (To Say Nothing of the Dog) -- Jerome K. Jerome