Dread Companion by Andre Norton
Dread Companion is my favorite type of Andre Norton novel. A lot of her novels have grim subject-matter -- orphans, refugees, war, crisis -- but her best novels add a weirdness to the darkness. She can create a twilight mood with the best of them, in which the uncertain light threatens to reveal inner monstrosity.
Dread Companion is a science fiction wrapper around a fantasy core. The opening is a very sharp depiction of social customs in the future history that Andre Norton used in most of her science fiction stories. Protagonist Kilda c'Rhyn is the child of a Survey scout father and a mother from a trading family. Her parents entered a standard three-year marriage contract common for Survey scouts, who can't stay in one place for long, and Kilda was raised in a creche after he left for his next mission. She never saw either parent again. The children of such "cross-births" are usual male and can qualify for government service, but as a female she's blocked from this career path. (An interesting bit of feminism in this 1970 novel.) Instead she uses her mutant outcast mentor, Lazk Volk, to find her a governess job with the wife of a government archaelogist and their teenage daughter and son. The husband has been assigned to the planet Dylan, where his job is to judge whether the mysterious ruins ("it might once have either had native inhabitants or been a colony of one of the Forerunner race"), and they travel by spaceliner to meet him there.
The daughter, Bartare, is a difficult personality who is clearly hiding something. Both she and the boy, Oomark, refer to a Lady who is invisible to everyone else, and Oomark shows signs of extreme anxiety regarding Her. Eventually all three characters are transported to an alternate reality, which even the characters themselves compare to Faerie. Both Ooomark and Kilda begin to transform into strange creatures, although Kilda resists it. This theme of becoming alien is also quite common in Norton's novels, and it's where the weirdness really starts to enter the story, as Kilda finds her toes becoming rootlike and attempting to dig into the ground for nourishment. This is yet another rite of passage story, which is usually what Norton's young adult novels are, and Kilda is another orphan/outcast who finds herself thrust into an existential crisis in which she struggles to find the inner grit, the external resources, and the allies to help her survive.
Because of the reference to Forerunner races early on, I kept expecting the strange twilight world they find themselves in to be rationalized in terms of a lost superscientific technology, but in fact the story becomes a straight magical fantasy in the long middle section. However, as in Ordeal in Otherwhere, the alternate reality and magical powers of its inhabitants have rules that do give even this part of the story a science fictional cast, and Norton is particularly good at depicting the shifting transformations of the characters from human to alien/supernatural and back, depending largely on the food they eat but also on certain talismans of power. As is also typical of her work, there are several contending factions in a complicated array, with the beleaguered protagonists stuck somewhere in the indefinite middle of the conflict between multiple sides. My one complaint about this section is that the character reactions seemed a little repetitive at times.
What really lifts Dread Companion above the run of Norton's mill is the ending, and I don't think I can really discuss this without SPOILERS SPOILERS SPOILERS. Of course Kilda and the two children and another character whom they meet in the alternate reality escape back to the planet Dylan. But like other visitors to Faerie, they discover that a great deal of time has passed while they were away. Basically the world they return to has been completely transformed. An alien invasion destroyed civilization in that sector of the galaxy, and Dylan was evacuated, leaving only a rump colony. Everybody the characters knew is long dead, and their choices are either to join the rump colony and breed children, or ... what? There's another option involving the stranger they met in the alternate reality, who was there even longer than they were, but while it gives Kilda some hope for the future, the uncertainty of the situation she faces is, if anything, even greater than her uncertain prospects at the beginning of the novel. Now all four characters are orphans of a sort, having lost not just their families but pretty much everything else as well. We've seen Kilda survive a terrible ordeal, so we expect she'll do okay, but nothing has gotten easier, despite her rite of passage. This is pretty powerful stuff, and I could easily see this one as a direct influence on C.J. Cherryh, who has acknowledged a debt to Norton. I also think Dread Companion is another great title, invoking an anxious, threatening intimacy.