Though set in a Celtic fantasy world reminiscent of The Dreamstone and The Tree of Swords and Jewels, this book is more like Cherryh's "human among the aliens" SF, such as Hunter of Worlds. The aliens, in this case, are the Sidhe. These aren't J.R.R. Tolkien's elves. They are like forces of nature, unmoved by, perhaps even unable to comprehend, the things that are important to mortals. Yet one gets the feeling that they are fond of Caith, in their inhuman way. Poor Caith.
Cherryh has a knack for hard-edged fantasy, and it's on fine display here. As in Rusalka, magic works in this universe according to rules and limitations that make it seem almost like science. The characters also have more of an edge than you commonly find in fantasy. Cherryh's characters are always interestingly flawed, but Caith and Dubhain are especially so. Caith has had an exceedingly rough life, and has good reason for carrying the chip on his shoulder he does. He's basically a decent guy, but his hot temper and stubbornness often get him into trouble. As for Dubhain...well, he's a dark Sidhe, a pooka whose job it is to take horse form, lure men onto his back, then drown them. Talk about having a dark side! Nevertheless, he has a sort of elemental innocence that makes it easy to forgive his mischief.
What Pat Nussman called "the magic circle" - a situation where a pair of characters are forced to rely on only each other for trust and friendship - is quite literal here. Caith has been damned by Faery, more because of the family he was born to than anything he did to deserve it. He avoids human company for fear of bringing his misfortune on others. Dubhain is similarly damned for the crime of doing a good deed. In a moment of weakness, he rescued Caith rather than drown him, and for this failure, he was bound to Caith by geas. They are each other's punishment...but also friends, as much as human and Sidhe can be. Dubhain is wicked and feckless and not entirely trustworthy, but humans need companionship, and he's all Caith has - the only being it's safe for him to be with. Dubhain is a loyal friend, as much as a dark Sidhe can be, but he's not human, and doesn't live by human rules. He can't give gifts without strings attached; every kindness must be balanced by a cruelty, however small. It's just his nature, and Caith understands this, even as he curses it. They are devoted to each other, but neither will ever admit it. They taunt and tease each other incessantly, using sarcastic terms of endearment more suitable for lovers than friends. Yes, it's incredibly slashy.
This book is a sequel to "The Brothers," a story from Cherryh's 1986 anthology, Visible Light. (The story can also be read in the omnibus The Collected Short Fiction of C.J. Cherryh.) However, the book stands alone. The key points from "The Brothers" are covered in Faery In Shadow, in flashback. (The events of Faery In Shadow occur five years after "The Brothers.")
Faery In Shadow is one of my favorite books of all time. If the Sequel Fay appeared and gave me one wish, this is the book I'd want a sequel to. Alas, barring such magical intervention, it's not likely. Faery In Shadow didn't get much notice when it was originally published. Cherryh says that her publisher thought it was too dark and depressing. Many reviews and readers have said the same. Part of the reason for this is that these days, people just expect fantasies to be fluffy. (Curse you, Piers Anthony.) And part of it is that, like many Cherryh books, the ending of this one wasn't really meant to be "The End," it was "To Be Continued." Caith does still have some serious issues at the end of the book. And anyone who was hoping for him to settle down with the pretty girl and live happily ever after is going to be disappointed. But viewed through a fannish eye, the ending is perfect. Caith and Dubhain are together, still bound by geas, and presumably heading for further adventures. Frankly, I'd have been disappointed in anything else, since the deliciously dysfunctional relationship between Caith and Dubhain is the heart of the book.
Faery In Shadow FAQ at the official Cherryh site - C.J. answers frequently asked questions about Faery In Shadow. Some great stuff. She says she's planning a sequel, and I haven't completely given up hope, but from talking to her in at cons and online, I gather it's not likely.
Cherryh's Good-Looking Guys - page of portraits of Cherryh character, done by C.J. and her friend, Jane Fancher. Includes a nice sketch of Dubhain by the author.
Shejidan - The name of this site is from the Foreigner series, but it's a general Cherryh site, with art, discussion, etc. IMO, the best Cherryh site on the net.
Rolling Naked in the Snow - Wow, look! Someone has committed Faery In Shadow fanfic. (Fear thee not. Despite the title, it's not one of "those" stories.)