So you cast a few spells over a period of years and are satisfied with the results. Now you're looking for a book that will tell you, not what magic is or how magic works, but why it works.
This one may not tell you why it works as much on why it might work. However, unless you're British or have a familiarity with the language scientists use in quantum mechanics and string theory (or both), you might find it a difficult read. It may only have 120 pages, but what would ordinarily take maybe a few hours to read could actually turn into a few days or weeks because of the intellectual treatment of the subject.
If Hogwarts actually existed, then consider this one of the textbooks they might assign for intermediate or advanced students in magic theory. It starts out addressing the subject of sorcery, but if you don't watch it, you'll find yourself constantly reminding yourself that you're not reading a book on science, but on sorcery. You'll be lost and so over your head in scientific jargon before you know it; a rather uncomfortable place to be, especially if you're the type who hates science because of its cold, materialist, mechanical viewpoint of the universe, and wishes it would take a long walk off a short pier. But if you can forgive how deep it goes into the history, personalities, theories, and experiments that might support why magic might work, you could come away with a whole new way of looking at the possibility of the existence of magic. You could also be tempted to consider selling back almost all your magical cookbooks and paring down your library quite a bit.
I would definitely encourage anyone curious about any possible explanations for why real magic could work to read this book, even if it is kind of heady in its approach. Hang in there; you'll get through it. You might be glad you did. ( )