When King Alfonso X of Castile and León commissioned the scholars in his employ to collect and translate the wisdom of the ancients for the benefit of his kingdom, their search took them far beyond the familiar works of Aristotle and Virgil. One of the texts which emerged from their labors was the Liber Razielis, or Book of Raziel, a seven-volume compilation of cosmological lore and efficacious magic supposedly revealed to Adam by the angel Raziel. As if that were not enough, Alfonso's translators appended an additional nine texts to this anthology, supplying further explication and operationalization of the material presented in the Razielis proper. The Book of Theyzolius is one of those nine appendices. This treatise expands upon the discourse of magically powerful names found in the Razielis, providing extensive lists of these along with practical instructions for using them to command angels, demons, winds, and the spirits of the dead. The present volume includes the first ever complete critical edition of the Latin Liber Theyzolius, based upon all of the extant manuscript sources, as well as an English translation of the text and a selection of excerpts from the Liber Razielis which further elucidate the Book of Theyzolius and its methodology.