Rule is, first and foremost, a transition novel between the fourth and sixth books. If you don't have that in mind, the book probably doesn't seem good. The whole thing is a bundle of plotlines that are brought up and left unresolved and not much definate happens the whole time, unlike the previous books which can stand alone. That isn't good for the volume itself. However, it does serve to set up for the final book quite well.
Moreover, we finally get to see what I wanted since book one-the three siblings finally interacting on a regular basis. That was missing in Dragon Strike, which made the book a letdown for me. Here, though, they maneuver around and bounce of each other. The book has a ten year gap from the last one and by this time they've gotten used to each other's existance, but even with the lost time theres still alot of character clash. The Dragons personas, and watching them interact and change over the course of the book, is the high point-action scenes are less prominant, and I barely miseed them. One point had Auron planning a party and I was actually enjoying myself.
Unfortunatly, it's held back by numerous flaws. The editing is pretty bad. There are several typos, weird perspective changes, and overall a bad flow between many scenes. Often I got the sensation that the characters had forgotten what just happened in the last section, as something big would come up and then be dropped. Moreover, it's the shortest book yet and feels even more rushed then the others, with scenes feel shortened or excised(summarizing a conversation, for instance, rather then actually recounting it). If Knight had taken the time to slow down, elaborate, and add some filler, it would have been much better. As it is, I feel that I have to make do with a bare minimum offering, and only the characters keep it up. The one good thing is that unlike the other books, which started out well and rushed in the last half, Rule does the opposite. The "finale" comes 3/4ths through the book, leaving a long, well connected epilogue section that was probably the best part. Except for the very end, which focuses on Wistala's....ahem, personal issues and they overshadow the larger picture, in a detrimental way. That shouldn't have been the last scene IMO, and that's all I'll say.
Finally, the book seems to go out of it's way to reintroduce characters from previous books and wrapping a few old things up, or showing them in a new light. While that's fine, it also goes out of it's way not to introduce anything or anyone new either-everything's been seen before, and the world and cast does not expand in any noticable way. However, some of the "in a new light" stuff borders on character derailment and in one case, retcon. More bad editing, I guess.
The gist of it is that Rule is a worthy continuation of the series, but pays for the character interaction in terms of editing. If it had been longer and more flushed out, it probably would have been the best book. Nevertheless, I eagerly await the final book, which unfortunatly has been delayed six months.