The River of Wind — Kathryn Lasky

Coryn, Soren, and the chaw of chaws ride a river of wind to discover a realm of perpetual winter where dwells a society of mysterious owls, the likes of which has never before been seen.

Meanwhile, journeying in the known kingdoms, Primrose and Eglantine stumble upon a plot by Nyra to kill the young king and his companions in a distant snowy land. Too late to get help from the tree, the young Guardians must find the strange new realm on their own to warn Coryn and the others. Even if they make it in time, will a handful of Guardians and peace-loving monks be a match for Pure Ones bent on murder?