Three Books of Occult Philosophy (De Occulta Philosophia Libri Tres) is Heinrich Cornelius Agrippa's study of occult philosophy, acknowledged as a significant contribution to the renaissance philosophical discussion concerning the powers of ritual magic, and its relationship with religion.
The three books deal with Natural (or Elemental), Celestial and Intellectual magic. The books outline the four elements, astrology, kabbalah, numbers, angels, God's names, the virtues and relationships with each other as well as methods of utilizing these relationships and laws in medicine, scrying, alchemy, ceremonies, origins of what are from the Hebrew, Greek and Chaldean context.
These discussions were common amongst other hermetic philosophers at the time and before. In fact, Agrippa's interpretation of magic is similar to several contemporary authors’ synthesis of magic and religion, and emphasize an exploration of nature. Compared to the many grimoires of the time, these books are scholarly works, and as such these books are still read by those you study the occult today, both as primary sources of renaissance occultism, and as inspiration for modern occult works.