I think The Grey King has always been my favourite. It follows on very nicely from Greenwitch in some ways, because Greenwitch started to show that the Light didn't have all the answers and needed Jane's humanity and emotion. The Grey King is very different, but is still a book about vulnerability and complexity and pain and things just not being as simple as we would like them to be.
Will is vulnerable from the start, as a result of having been ill and lost such important knowledge. He's vulnerable for being a stranger in a strange country, on his own, away from Merriman's guidance. And the Light is vulnerable because this is Will's task, Will's and Bran's, which means the Light must rely on the apparently weakest of the Old Ones and a rather ambiguous human whose choice is essential.
Bran is an interesting figure—for many reasons, of course, but especially because he is the first non-Old One who gets entrusted with Sekrit Light Bizness without having first proved himself the way the Drews did, by actually participating in the quest. He does go on to participate, of course, but the fact that Merriman seeks him out and tells him pretty hard-won knowledge so that Bran can prove himself to Will is pretty unusual. Will himself seems to feel a bit uncomfortable with Bran, though he does trust him. Bran likes being different, likes being set apart and a cause of unease. Will, I get the feeling, much as he loves being an Old One, would love to be one in the context of a team, a family, to belong to something bigger instead of pitting himself solo against the odds. That's precisely what he has to do in this one, which brings me back to that vulnerability thing.
Of course both Will and Bran are hurt—Bran more so, I think. But it is that acceptance of their vulnerability, of the need to choose to love, to be able to be hurt by who we love and to be hurt by their loss, that makes so much happen in this book. Cafall's sacrifice; Owen Davies's choice to love and lose Gwen and care for her son; Bran's loss of Cafall that makes him angrily reject the Light only to risk himself again so that Pen isn't needlessly sacrificed as well. There is so much pain in this book, and yet it isn't pain without reward.
And then there's John Rowlands, who is even more special than Bran in some ways, because he is a perfectly ordinary human and yet he knows what is going on without being told; in fact, he actively does not want to be told any more than he's already figured out. He helps Will anyway, and it's through Will's discussions with John that we find out much more clearly what the Light are for and why the Dark must be fought. It is John who spells it out, challenges Will and the Light for their mercilessness and single-mindedness in pursuit of their goals. That in some ways, they do not seem so different from the Dark because from a human's perspective, both Light and Dark can wound humans deeply and permanently; both Light and Dark can use humans for their own ends, no matter whether those ends are ultimately good or evil. And it is to John that Will explains it, at last. I'm going to quote here, because this is a pretty important passage.
"I understand what you are saying," he said sadly. "But you misjudge us, because you are a man yourself. For us, there is only the destiny. Like a job to be done. We are here simply to save the world from the Dark. Make no mistake, John, the Dark is rising, and will take the world to itself very soon if nothing stands in its way. And if that should happen, then there would be no question ever, for anyone, either of warm charity or of cold absolute good, because nothing would exist in the world or in the hearts of men except that bottomless black pit. The charity and the mercy and the humanitarianism are for you, they are the only things by which men are able to exist together in peace. But in this hard case that we the Light are in, confronting the Dark, we can make no use of them. We are fighting a war. We are fighting for life or death—not for our life, remember, since we cannot die. For yours."
"Sometimes," Will said slowly, "in this sort of war, it is not possible to pause, to smooth the way for one human being, because even that one small thing could mean an end of the world for all the rest."
...[John says:] "It is a cold world you live in, bachgen. I do not think so far ahead, myself. I would take the one human being over all the principle, all the time."
..."Oh, so would I," he said sadly. "So would I, if I could. It would feel a lot better inside me. But it wouldn't work."
Probably just as well I'm not an old one, because I'd be a lot more likely to behave like John. And the Light does need people like John, but that's not their job. They're not allowed that.
Not an easy book, not for any of the characters in it.