The Language of Stones by Robert Carter
Tolkien and TH White parallels abound in this huge slice of mythic fiction set in 15th century England. England is about to enter an era of unimaginable divison and destruction: the Wars of the Roses. The wizard Gwydion and his young apprentice Willand must undertake a great quest to save the land from devastation by terrible warfare between rival factions for the crown. On the one side is Duke Richard of Ebor and his sons and allies; on the other the sickly King Hal and devilish wife Queen Mag and her sorcerer Maskull, Gwydion's evil counterpart. England is a still-magical land, crisscrossed by lines of power which run from one great sarsen stone to another. The lines must be paired (male and female, peaceable and warlike) to maintain the Balance - of Nature, of magic, and the equanimity of man - and so protect the land. But the purity of the lines' magic has been subverted over millennia: first by the Slavers who came and laid their own grids of stone roads, tearing up the stones from their rightful places to build their cities and temples; and now by the sinister Sightless Ones, whose joyless, ritual-bound religion has displaced the ways of the true folk of the land.