I think middle books are the easiest to write. You don’t have to introduce a whole world, like you do in a first book, and your reader will know at least some of your characters. So you can play around with the details, expanding what we know about the world or turning things on their heads. You can introduce new characters and themes if you really want to in a middle book, but there’s a lot less pressure.
The hardest books to write are the last in a series. You have to wrap everything up, which is quite daunting. It’s sort of like that moment when a juggler stops juggling, and has to catch all those balls with only two hands. That part always look a bit gawky compared to the elegant balls-in-the-air part, and I think last books in a series can have that same sense of ungainliness.
Maybe that’s why writers have tended toward longer and longer series. After all, a trilogy only has one middle book, but a ten-book series has eight delicious middle books! And once you’ve written that many, it’s even harder to wrap things up, so I can see why some writers (you know who you are) seem to go on forever.
