I've often felt that the series lost its free spirit and lightness and became incredibly personal towards the end. Maybe it always was incredibly personal. I don't know if it's true or if it's just Douglas's depression. The time with Fenchurch feels almost like a stolen glimpse of a very private moment.
Some points also to Eoin Colfer who didn't make a total hash of the sequel book, which I imagine is a very difficult thing to approach and would inevitably upset a lot of people.