These are two teenage fantasy novels, the start of a series of four. Wiki has an article if you want details. They are lightweight fun; Miranda was given them by Becky for Christmas, though I think she hasn't read them yet. They are lightweight in the way that, say, the Owl Service is not. I'm likely to finish off the series.
They most remind me of Dragonflight, but also a host of others (the Galadriel /Lothlorien bit is a touch blatant); the story is not strikingly original but its fairly well told. One thing that always slightly winds me up (because I've done a bit of travelling on cold weather, and I know how hard it can be, with modern kit etc) is how the protagonists always survive extensive journeys across frozen plains or lofty mountains despite minimal equipement. But that's just me picking flaws.