Magical Theory is a book written by Adalbert Waffling.
It can be bought at Flourish and Blotts for two galleons. It is a required first year textbook and therefore cannot be traded unless more than one copy is owned.
"Magical Theory" is first discovered in Harry Potter and the Philosopher's Stone, chapter 5, Diagon Alley, "Arriving at Diagon Alley".
|| Magical Theory, by Adalbert Waffling, is a required text for Hogwarts first-years.
|| All you will ever need to know about what makes spells work.