Donaldson, Stephen R. Great God's war #1

Two kingdoms, engaged in a brutal, unrelenting conflict. Magisters on both sides wield their powerful magical decimates--of fire, of wind, of drought, of lightning, of earthquake, or of pestilence--until both kingdoms are reeling. But now, something has changed. There is a seventh decimate, one that blocks all lesser sorceries. And Prince Bifalt believes his enemy has used it against them. Unless the prince can gain this power for his kingdom, they will be helpless against the enemy's next onslaught. Legends speak of the Last Repository, a mysterious library that holds all the sorcerous knowledge of the magisters. And it is the prince's only hope