This series is complex, with lots of moving parts. Normally I re-read early books before reading the latest in a series, and I strongly recommend doing that here. I wasn't able to, and the provided "What has gone before" summary was limited (and seemed to lose steam half-way through its few pages). You can jump straight in, but it takes a while to remember and re-orient.
Watcher of the Dead is a straightforward continuation of the series. We learn more about our hero Raif, and, separately, his engaging sister Effie. The plot moves forward in fun if not terribly surprising ways. One of the things I enjoy about Jones is her willingness to defy happy-ending expectations and maim or kill key characters, and some of that goes on here.
A few mysteries are revealed, but not very many. This book is primarily about moving the story along. I was disappointed to finally reach the Heart of the Sull, and find it not very different from anywhere else. This was surprising from an author who normally does well in creating murky mystery and revealing excitement. The same is somewhat true for some of Raif's travails. While interesting, it didn't feel like a top rung effort, and there's an element at the end that seemed to come almost out of nowhere.
Nonetheless, this is a very enjoyable sequel, and the overall story is still exciting and fairly fresh. All in all, well worth picking up for readers of the series. Definitely not the place to start if you've not read the prior books. (Though if you haven't, I recommend you do). I haven't seen anything about Ms. Jones' plans, but I anticipate that the next book (or at most the one after) will be the end of the series. Much more than that (without a fresh infusion of energy), and I think the story will have stretched too far.