Earth's magic : book three in the new magic trilogy

Heather and Merlin are working with King Arthur to unite Britain following a terrible nuclear devastation. Merlin's sworn enemy, Morgan Le Fay, is also organizing her forces, hoping to take over the world once and for all. Earth's only hope is for Merlin to find his father, who has been trapped in an enchantment for two thousand years. The problem is, Merlin doesn't know where to look, and worse, doesn't even know what form the enchantment takes....