In retrospect, I suspect I should *not* have been surprised by the ending of the book; in a sense, the ending--and the coda which follows it--were set up in the very first book in the trilogy, "Cosmonaut Keep." The central theme of this book appears to be irony, from first page to last.
MacLeod has created a bizarre universe, populated with many different creatures, including saurs, krakens, selkies, and, perhaps the most alien of all, the eight-legged Multipliers. There's a lot of intriguing ideas jammed in here.
Unfortunately, all those ideas, in a book this short, mean that a lot of characters get short shrift. Likewise, the book isn't long enough to stand on its own; why certain characters behave the way they do doesn't really make sense unless you've read the previous two books. Thus, the series ends leaving a lot of questions (not the least of which is why the book is written in the present tense when, and only when, Matt Cairns is the viewpoint character).
All in all, though, if you've read the first two books, you'll probably want to read this one just to see how it ends. If you haven't, start with "Cosmonaut Keep" and "Dark Light" before reading this one.