I love reading books again and again. It's like coming back to old friends. And you will always notice something new or make a new connection that you hadn't thought of before. Even if the world in that book is always the same, the reader is not, thus the story will always grow.
Or a freaking GLOSSARY!!--No but seriously, its terrible. The only time i like it is when its a chapter to the next book in the series. But even then, you read it and still have to wait to get the freaking next book (unless its out already)