This text has a twofold project. One, Couliano wishes to elucidate what he sees as the defining charachteristic of Renaissance Magic, that of "Eros," and also to account for the shift in thinking that reportedly heralded the "decline" of magic in the sixteenth and seventeenth century.
The "eros" of Renaissance magic started out with optical theory and other medical concerns with Aristotle (and perhaps Plato), who held that there was a substance called the "pneuma." In Aristotle's thinking, the pneuma was a substance that was located as a thin shield around the body. In Stoic medical theory, this became a substance commesurate with the "soul" or "spirit." This substance was a "prima materia," a fundamental substance that contained the physiological ability to transmit information to the senses, especially the ocular sense. The heart was the center for a generational organ that in turn centered the pneuma, This pneumatic organ was called in Greek --- the "hegimonikon." Forming images in the pneuma for sensory transmission was necessary before a person could percieve something or someone. Through the works of late antiquity, such as the Corpus Hermeticum and medieval physicians such as Albert the Great, the doctrine of the pneuma became common discourse and was incorporated into popular culture such as the courtly love tradition. Taken by the bishop Synesius's (d. ca. 415) synthesis of previous pneumatic doctrine and courtly love practices, Ficino develops a universal doctrine of the relation of man to the universe through Eros mediated by the Universal and Particular pneuma. While mentioning Pico della Mirandola as a sparring partner of Ficino, the main emphasis in this narrative turns to Giordano Bruno, whom Couliano believes modified and perfected this doctrine in terms of personal manipulation and excitation through the powers of Eros.
Couliano, in the last part of the book, strives to develop an alternate account of the "fall" of magic by highlighting the role of the Reformation. Having defended the notion that the Renaissance was about a revival of pagan culture, he in turn emphasizes the role of imagery and "phantasy" in the doctrine of the pneuma. The Reformation and the Counter Reformation were primarily about the eradication of pagan culture from Christiandom. As such they were about the eradication of imagery, manifested in terms of Luther's accusations of Catholic "magic" in the Eucharist, iconoclasm, the witch hunts. For Couliano the witch hunts are a social counterpart to the eradication of religious-magical imagery--- both are manifestations of "human phantasy." When "qualitative" statements become suspect (as they involve imagery) then strictly "quantitative" science becomes the only legitimate route for knowledge. When these scientists wax inductive, they are threatened by the Church(es).
Better than Keith Thomas's 'Religion and the Decline of Magic" but if you're looking for the real explanations of how Renaissance Magic worked, then you should read D. P. Walker's "Spiritual and Demonic Magic" instead.