User Stories Applied: For Agile Software Development by Mike Cohn
"User stories" and "requirements" are not always mentioned in the same sentences, except erroneously. So let me state at the outset that user stories are not requirements, they are placeholders for requirements. I am happy too see that Mike Cohn endorses this view, and got some way to bridging the gap between stories and requirements. He also gives some good advice of selecting the personas (my word, he doesn't call them that) for the stories.
User stories are often a weak link in the agile process, and I suggest strongly that anyone using any agile process read and digest this book.