This series continues to improve, although not a lot happens in this one; It seems more of a bridge between the last book and the next one, it is still nicely self-contained and moves the plot forwards. The ending is a little less neat than the previous two books, there's more left clearly unresolved than in those two, but it really sets the stage for the next one very well, and pays off a lot of stuff that's been building since the first book.
A lot of what happens in The Rival feels fairly inevitable - we knew that Black King would arrive eventually, and there was no way that Blue Isle could hold off the full force of the Fey army, but that doesn't mean the story is over, it just changes the focus. It's no longer about keeping the Fey from taking Blue Isle, it's now about how to deal with the fact that they're there.
I have some idea of where the next book may go, but there are a lot of little variables that could have unexpected consequences, and in this series it's hard to know what's going to be important later or which characters will suddenly die with their plans incomplete.