Oh wow... don't know how to break it to you but you stopped right before the series gets really interesting. Up until 7 or so it's kind of episodic, but after that every book is clearly advancing a larger plot arc (i.e. the kids aren't transitioning between guardians at the end of each book anymore.) You should read the rest! :)

Oh -- I haven't read Letters but might very well pick that up today along with The End... we'll see how strong my willpower in the bookstore is.