My favorite books, movies, and television shows are those that have interesting, complex characters and storylines. Lost, Fringe, and Game of Thrones are but a few programs that peaked my creative interest. The first and third are also examples of how difficult it can be to close out a series (Fringe, on the other hand, was near-perfect).
It is easy to criticize shows for failing to properly end all storylines and conclude character arcs (here's looking at you, X-Files). Books are not immune to this. The Harry Potter universe is so vast and rich in detail that numerous questions were bound to be left dangling.
Even if all of a series' threads are neatly tied up at the end, that is no guarantee that it's satisfying.
I am about a third of the way through the first draft of Book 3 in The Brass Machine trilogy. The first two books introduced a plethora of wildly different characters with their own wants and needs, as well as a growing mythology. The final book continues that, starting out with a character only mentioned but never seen in the previous two (I have yet to pick up from where Fear left off or even mention Prince Kitsune).
Writing is slow going, and not just because the weather has been gorgeous. While building upon the world I've already created, I am also attempting to weave storylines, tie together details, and carry several characters through to inevitable (and hopefully satisfying) ending. I want to explain how everything fits together while not overexplaining. The world of Kitsune and Myobu is one filled with magic, wonder, and the unknown. To explain everything concretely would be a disservice.
Closing out a series can be difficult, and I'm beginning to understand why (again, X-Files, you're original finale was not good, but the revival's was complete trash). I'm taking my time to get Book 3 done right. The story is exciting to me so far, and I can't wait to share it with everyone!