Lynn Flewelling's Stalking Darkness is the second book of her Nightrunner series -- which is, apparently, not a trilogy.
As I was reading this book, I was thinking to myself, "My, this plot is progressing amazingly quickly. What's possibly going to be left to happen in the third book?" And then I got to the end, and it looked like the answer was "Nothing much, really," so I went downstairs and grabbed the third book in the series off the shelf. Much confusion was resolved, as the author's foreword made it clear that the Nightrunner series is meant to consist of an initial duology (of which Stalking Darkness is the second half) and then an indeterminate number of standalone stories. The sentence "This is not a trilogy," appeared three times in succession, and everyone knows that something repeated three times is true.
I'm quite pleased by this. Flewelling probably could have padded the plot out to a trilogy without much difficulty, but as a duology, it's got some snap-snap pacing going on, and was a fun read.