The History of Magic, while being a history, is also the dominant text on magical theory, and has been since its publication in 1694. Written by Sir Isaac Newton, it laid out the known history of magical development based on secret records within the British Order of Magi left over from when it was the Order of the Veil. While it laid out the known history, Newton also theorized on the nature of magic and why it works the way it does, as well as applying mathematical observations to several magical applications. He also theorized that if magic indeed flowed from another reality, then it is entirely possible that there are other realities as well parallel to our own, possibly populated. These theories and applications form the basis of magical research to this day.
Theories on the nature of Magic.
Structure and Belief
Newton theorized in the book that during the first days after the sundering of the Veil, the supernatural energies alien to our reality required structure, and they took that structure from human beliefs. Because the tear in the veil occurred in Europe, European belief systems formed the backbone of most magical laws. The basis of arcane magic for instance he theorized was based on transmutation, an alchemical process. Alchemy being mostly based on Aristotles' classical elements. Faith based magics were easier to explain in this way as most recorded Christian miracles involved healing the sick, inspiring the hopeless, and smiting the wicked Pagan resurgent faiths had clear domains for their gods, which specialised the abilities of their clerics.
Newton based his theory that magical structure was based mostly on European beliefs because until Europeans began trading heavily outside of Europe, most nations and peoples across the globe only had access to either faith-based or instinctual magics.
Divine Right to Rule
One of the more controversial observations was his theory that the belief in the divine right to rule by the nobility was actually enhanced by magic tying increased leadership ability and charisma to noble bloodlines. However with the constant infidelities by the nobility these traits have been passed on to the common folk over the decades which he theorizes is why there seemed to be an increase in effective civilian leadership. These theories became the basis of many republican movements in the last century or so.