Impossible, really, to choose a favourite book, but I suppose it would be Lewis Carroll's Alice's Adventures in Wonderland. And this is my favourite quote from the book:
"In that direction," the Cat said, waving its right paw round, "lives a Hatter: and in that direction," waving the other paw, "lives a March Hare. Visit either you like: they're both mad."
"But I don't want to go among mad people," Alice remarked.
"Oh, you can't help that," said the Cat: "We're all mad here. I'm mad. You're mad."
"How do you know I'm mad?" said Alice.
"You must be, said the Cat, "or you wouldn't have come here."
Often-quoted, much-analysed, but probably the best dialogue in the whole book!