The Pendragon's Quest — Sarah Woodbury

This refers to an out of print version of the book.

Although the dark forces of Arawn are defeated, the Saxons have massed on the border of Wales, aiming for nothing less than the complete subjugation of the Welsh people. Meanwhile, the fickle god Mabon, loose again in the human world, searches for the Thirteen Treasures of Britain. With each one he collects, his power grows. 

With the other gods forbidden to interfere in the human world, it is up to Cade, Rhiann, and their companions to stop Mabon and the Saxons. 

And time is running out.