Firstly I just want to mention that this story is quite simply fun to read. Despite all the deep and meaningful stuff that I'm about to go into concerning what I read between the lines of this fantasy adventure, the book isn't difficult, and is paced beautifully. The story is simply filled with crowning moments of awesome. All this other stuff I'm about to talk about, a reader can safely ignore and still enjoy the story. The world of Keltor is authentic, complicated and beautiful; a broken utopia which the two main characters struggle to navigate without creating any disasters in their wake, though they frequently do.
That reminds me of one thing that might detract for some, though I liked it: the conflict in this story is to a large extent of the political or manipulative kind, rather than the standard fantasy fare of pitched battles and violence, either physical or magical. There is action, but mostly the really cool stuff is when Ember or Glynn somehow manage to weasel or finesse their way safely through the politics and deception that they're frequently surrounded by.
So. It occurred to me recently, what sets Isobelle Carmody apart from other authors. When I think about other modern authors I know of, how many dare to comment on and criticize today's society through their fiction? I just started reading Jane Eyre (I know, not a likely comparison), and Charlotte Bronte referred to Vanity Fair in the preface, which I read a while ago. One thing that I noticed about both William Thackeray and "Currer Bell" was that they both commented on the society of their time. Very few modern authors I can think of care to comment on today's society. I've read many books that mirror our society, sometimes the more seamy parts of it (as if that creates depth by itself), but you read between the lines of many such works, and just come up with nothing more than a soulless reflection. Carmody, in this series and in Alyzon Whitestarr particularly, but also in her other works, creates a compassionate subtextual commentary on the common beliefs of today's society for her YA audience to reflect upon if they're so inclined.
It seems perfectly likely to me that a strong and selfless character like Glynn would be largely shunned by today's society, and that the weaker, more selfish Ember would get all the attention. Apparently many of Glynn's difficulties are orchestrated by the "watcher" who directs the story, but she seems like many of the teenagers I knew in school who were ostracized: socially clumsy, but gentle, more likely to endure torment than to inflict it, or even to retaliate against it, even if they were quite able to; quicker to defend a friend than themselves. She's a slight Mary Sue, but it seems to me that she's only enough so to create Carmody's subtext. Meanwhile Ember's journey of self-discovery could be looked at as hyperbole created to demonstrate the many aspects of the conflicting forces of despair and hope that we all have within us at times.
Which is probably the major theme of the story: hope vs despair. It's nice to have a fantasy story where the forces of good and evil are a bit more than two not very distinguishable groups, one of which, much like Pinky and the Brain, have decided to TRY TO TAKE OVER THE WORLD! Or destroy it... whatever. It's nice to have characters that are fighting something that anyone and everyone has encountered from time to time. It seems to me that in the figurative sense at least, the Song and Chaos Spirit are essentially real.
One last thought: if you happen to read Alyzon Whitestarr, I suspect that it's meant to be set in the same universe as this series.
2018 edit: the trilogy is still unfinished! Grr. Something I've thought is that the book itself is like one of those tapestries that seers in the books create. They have these visions during which they create the tapestries, which become prophecies about the future. Once the vision is over they stop weaving and leave the tapestry unfinished. These tapestries are nonetheless highly prized for their rarity and beauty, which is kind of how I feel about this book. Is that... a bit too meta of me?