The first book is hard because everything is new and you’re setting up the ground rules for your universe and the patterns you’re going to be following. In many ways the first book is the hardest part of writing a series. That’s because, if you’ve done everything right, you’ve had to do all the work of set-up and outlining where the overall story arc is going to go in that first book. For the ones that come after it’s like filling in the blanks. You know that you have to reach point C in the overall arc and that you want to have this joke reoccur or this character grow in a certain way. So, I very much believe the first one is the hardest.
Even so, though, there can come a time when a book further down the line becomes harder to write, not because you’re not sure what you want to do, but because you’re bored or growing tired of the characters or are already working on the next series in your head. I call this burnout and if you’re not careful it can make your last book not more difficult than the first but certainly more frustrating.
