Son of the Shadows is one of my favorite books out there, and I've read a lot; this is one I just keep coming back to. And yet, I find myself agreeing with much of the criticism other reviewers have posted; I think this book is great for a particular demographic but certainly not for everyone.
Technically this is a sequel to Daughter of the Forest, but Son of the Shadows picks up the story a generation down the road with new main characters, and thus it is entirely possible to read it as a standalone. As other reviewers have noted, Daughter of the Forest is based on a fairy tale but Son of the Shadows is not, although elements of Irish mythology still abound.
This is the story of Liadan, the daughter of Sorcha, the previous heroine, who at the age of sixteen finds herself with two major problems. One is keeping her family together: there's her older sister Niamh, who embarks on a disastrous love affair; her twin brother Sean, with whom she has an increasingly difficult relationship; her mother, who is seriously ill; her father, who left more problems behind in Britain than he thought; and of course a diverse set of uncles, all of them pulling her in different directions. The other plotline revolves around her love life: Liadan really has to work at it to find a more socially unacceptable man than her mother did, but she succeeds and the result is probably the most beautiful love story I've ever read. It's a story of forbidden love that's actually tasteful--by which I don't mean sexless, but that there are a limited number of poignant scenes rather than the characters constantly mooning over one another, making out or declaring their undying love every other page. It's also refreshing to read a romance that, despite being female-oriented and narrated in first person from her perspective, still includes a love interest who has a life of his own and takes it seriously.
Aside from writing a great romance, Marillier's strengths in this book include a lyrical writing style, a strong and resourceful heroine, a refreshing take on visions of the future (for once they're not inevitable), a vivid sense of place, and the ability to draw a reader in so that you really care about the family interactions and enjoy the slow pacing. I'll be the first to admit that the book isn't perfect, though: the romantic hero is a bit too quick to share his feelings, running counter to his character; the poetic tends to take precedence over the way real people talk; Liadan is maybe a little too flawless; and toward the end there's a deus ex machina of sorts, which makes much more sense after reading the next book, Child of the Prophecy, but really bothered me the first time through. But then, there's a certain beauty to the setting and characters here that makes these problems less egregious than they would normally be.
I'll be in the minority in saying that this is actually my favorite of the Sevenwaters trilogy; I prefer Liadan's relative freedom of action to the agonizing internal conflicts faced by Sorcha and Fainne in the other books. But even if you haven't read the others, if this one sounds remotely interesting to you, give it a try! If you only like it half as much as I do, it'll still be worth your time.