Rosemary Sutcliff 
The Light Beyond the Forest: The Quest for the Holy Grail 
Part II of the Legends of King Arthur

Rooted in folklore, medieval ideals of chivalry, and the last gallant strugglesof the British against the Saxon invaders, the legends of King Arthur have been told in song and story since the middle ages.
When Percival comes to Camelot and Galahad sits in the Seat Perilous, as Merlin foretold, the quest for the Holy Grail scatters Arthur »s Knights far and wide, bringing death to many and bitter disappointment to the great Lancelot . . .