The important distinction here is that magic is an art form, and, more precisely, the use of magic is essentially a performance, a coordination of a state of mind and the proper physical movements. There are certain methods which form the foundation of Hermetic magic, but magic still remains, to a certain extent, something which is a deeply personal understanding.
Compare this to, say, playing a musical instrument. Becauuse I'm a violinist, I'm going to geek the history of violin performance for a bit; bear with me, please -- I hope the parallels to Hermetic magic will be obvious. :)
Over the years, the art of the violin has more or less coalesced into certain "standard" procedures, and various methodologies for teaching the basic principles of the instrument. In general, the instrument must be taught one-on-one, tailoring the method to the quirks of the individual student. The result has been, in the twentieth century, several broad traditions, following from the methodologies of great teachers of the past hundred years or so.
Some of the great teachers wrote about their vision of how the instrument should be played. They provided exercises for the improvement of specific aspects of the art, training both body and mind. These writings did not always agree with one another, and in fact, over the course of a violinist's life, he would alter what he had been taught by his teacher by what he picked up from elsewhere, either from the writings of others, observation, or, more often, personal experience. The eventual artistry of the students quite often surpassed the masters.
In communicating the art of performance, though, much was left out. One can show the precise placement of fingers on the instrument, but that doesn't account for the peculiarities of the student -- unusually short or fat fingers, for example. One can talk in generalities about the way a musical phrase should be ended, and how to "sense" the "artistic" way of phrasing something, but ultimately this sense must still be developed through personal practice; the book can help, but it does not teach all.
Finally, in the twentieth century, videotapes have been made of great teachers giving master classes, or demonstrating their art. Yet, even modern technology doesn't show everything; while one can gain some insights, it cannot show the precise pressure that one must use to achieve a certain effect, or teach the critical self-discipline needed to be able to achieve a certain performance on demand. What is natural to the teacher, and an essential part of his art, may seem to be a trivial and unimportant individual quirk to the outside observer. You can watch Jascha Heifetz (probably the greatest violinist of this century) teach or perform all day, and still not manage to even approach his level of mastery.
Magic, I think, is much the same way. A book tells you the basics of the writer's thoughts and style in performing that particular Art. The magus studying the book must work through the exercises and formulae provided on his own, developing his personal approach. The books provide theories, which must be experimented with, and perhaps partially discarded by the learner. I don't think a magus spends a Season just reading through a book; I think it's a matter of active practice -- it's not like reading a scientific or mathematical textbook. A magus simply can't describe exactly what he does to perform something; everyone's thought processes are different, plus magical talent varies from person to person. Even a personal demonstration isn't necessarily much more helpful; though it can clear up some vagaries of physical action, it still doesn't account for the personal quirks.
If magic were completely consistent, there wouldn't be die rolls needed on spellcasting. :)
-- Lydia