Wavesong (The Obernewtyn Chronicles #5)

With the end of the wintertime that isolates Obernewtyn from the rest of the world, Farseeker guildmistress Elspeth Gordie again sets out for the lowlands. But she soon finds that not everyone welcomes the changes brought about by the rebellion. There is a traitor among the rebels—a traitor whose hatred of Misfits puts Elspeth in danger as she attempts to thwart an invasion of fanatical Herders.