A Crown Disowned (The Cycle of Oak, Yew, Ash, and Rowan)

is book #3 of The Cycle of Oak, Yew, Ash, and Rowan.
==Book Description==
A Crown Disowned is the third volume of the Cycle of Oak, Yew, Ash, and Rowan that began with To the King a Daughter and continued in Knight or Knave.
The earth shakes and splits as the forces from the North draw nearer. The Ice Dragon Riders are speaking to the land, and more fire mountains awaken in the Bog. Rohan seeks to join forces with Tusser, leader of the Bog-folk, as Queen Ysa raises an army to clear the Bog.
War draws closer until even the Queen cannot deny it any longer. Raids from the north increase, and, for the first time, the Riders of the Ice Dragons appear. It is time for the Queen to give up her game of pitting one faction against another. Four great armies are assembled to march under the same banner. Though they do not represent the Four Trees, they nevertheless see this as a good omen.
Many good men from all four armies fall in battle, yet the Great Foulness is still at large. Is the combined might of the four powers enough to free the land from evil?
(Description from SashaMiller.com)