"Abhorsen" is the final book in the trilogy about the Old Kingdom by Garth Nix. There is one more book about the Old Kingdom, but it's a collection of shorter stories, and so it stands slightly separate from the first three books. I haven't read it yet (whereas this was my second time through the trilogy) and I really look forward to getting it from the library!
In Abhorsen we rejoin Lireal and Sam basically in the middle of the conflict. Ok, so they are actually at moment of reprieve when we rejoin them, but the previous book (Lirael) and this one could be read together pretty seamlessly.
Lirael is the newly discovered Abhorsen-in-waiting. That means that it's her duty to learn all she can and assist in making sure the dead stay where they belong, in the realm of the dead. She is also a Remembrencer, the first in a long, long time. As such she can go into death and view the past. That's how she discovered the truth about her parentage, and how she will learn the only way to stop the coming terror.
Sam, the Disreputable Dog, and Mogget are traveling with Lirael and doing everything they can to help. King Touchstone and the Abhorsen are over the wall in Ancielsterre, but they too will help when they can. The final confrontation is drawing near. We learn more about what it is Nick has been helping to uncover, and the truth is more horrible than any of them had imagined.