Sorceress of Faith - The Summoning Series, #2

Another American woman is Summoned to Lladrana... With evil invading Lladrana, the Sorcerers must Summon an outsider to stop the insidious Darkness slowly taking control of their land and targeting the Sorcerers - draining their Power and destroying the Sorcerers themselves. Arriving in Lladrana, grad student Marian Harasta is stunned to find herself the center of a dispute among Sorcerers who want to augment their Power with her own.
She must quickly learn her new magical Powers-and decide who she can trust in this strange land. She has prayed for a miracle to save her brother-could Lladrana offer that knowledge, and can she somehow return to Earth with it? She knows she must offer the Sorcerers something in the exchange, and ridding them of the Evil Sorcerer is what they want, but how will she fare in her first magical duel? With unexpected help-and unexpected betrayal-Marian must determine where her true fate lies.