Trilogies, Series and since when is one book not enough

As I told you in my first post, I consider myself kind of a book worm. I love books and I think they are infinitely more interesting than movies (not to say they don't have their own flair). Basically because I think books have a way to get under your skin. When you read a book you are destined to spend more time with it than watching a movie for 2 hours and also you have this incredible liberty of constructing your own little world, characters, places etc. I've been a passionate reader since I was a little girl but I can't say that I have ever encountered such a phenomenon when it comes to trilogies and series of books. Now, I know that they have always existed but never in this kind of fashion in the mainstream. Come to think of it...