Ars Magica — Judith Tarr

Gerbert was a farmer's son in an obscure town in France, but his gifts of mind and intellect were so remarkable that even in the feudal world of the tenth century, he could rise far above his station. Princes and prelates courted him; emperors called him friend and teacher. He brought the lost art of mathematics back into Europe; he was an astronomer, a musician, a builder of strange and wonderful devices. In the end he reached the pinnacle of the world, a seat so lofty and an authority so great that he answered only to God Himself. But Gerbert was more than a simple professor of the mathematical arts, or even a prince of the Church. As a young student in Spain, guided by his priestly patron, he entered into the study of another art altogether, a hidden art, mastering mighty powers of mystery and magic. Magic, as every student of the art knows, has a price -- and the greater the magic, the higher the price. The magic that came to Gerbert was very great indeed.