The White Tree: The Cycle of Arawn:

Sixteen-year-old Dante is obsessed with learning to wield the nether, the powerful magic of the death god Arawn. But a century ago, Arawn's followers were scoured from the kingdom of Mallon. Their temples smashed. Their secrets lost. When Dante tracks down a copy of their holiest book, he throws himself into its study--and finds himself under siege in the city streets. Arawn's believers aren't dead. They're in hiding, and they want back their book. With the help of the nether and a loudmouthed bodyguard named Blays, Dante escapes into the wilds. But Arawn's army is ready to march from the shadows. As they move against Mallon, they hunt Dante relentlessly, in search of the book. And with his powers growing by the day, Dante finds himself used as a tool in the war against them. He and Blays are dispatched on a thousand-mile journey to assassinate the Arawnites' leader. If they fail, their homeland will fall.