Edgerton's Green Lion trilogy is excellent- this is book 1, and it does a great job of setting up the storyline. I love all her books.
The Kingdom of Celydonn has been at peace for 50 years, since the great wizard Glastyn conquered the Wild Magic and drove its monsters underground. But Glastyn has disappeared and his apprentice Teleri is only a young girl, not experienced in sorcery. But even Teleri can see the crops are withering. And monsters stalk the forests, and corpses have washed ashore. Even more frightening is the aura of evil magic that surrounds Princess Diaspad. Teleri's magic is untested- but someone must act, or Celydonn will be full of blood and darkness.