I, Guinevere — C.K. Brooke

On the winter solstice, a mysterious sword in a stone appears in the churchyard. Not even the mightiest of the village men can remove it, until fifteen-year-old Guinevere gives a try. The sword heeds the unsuspecting maiden, proclaiming the unthinkable: she is the blood of Pendrakon, heir to the vacant throne of Camylot. 

Guinevere never dreamed she was born royalty. Now, between apprenticing the eccentric wizard, Merlyn; swordplay lessons with an abrasive--albeit, attractive--boy named Lance; and clandestine, magical meetings with the formidable High Priestess of Avalon, Guinevere is swept up in a whirlwind of training and preparation for her monumental new role as future queen. But invasions by the barbarous Saxyns and visitations from mysterious dark forces constantly warn that she may be in over her head. Can Guinevere defend the kingdom from the darkness and deception that threaten to seize it? Despite her doubts and the sinister forces working against her, can she harness the power to wield Exkalibur and rule the realm? Or is Camylot already destined to fall?