It is a book in which are contained spells considered "forbidden" because they are extremely powerful and dangerous so only a trained wizard must use. Because of the great power of these spells, the individual power of a wizard may not be enough to use them properly for what is required to use it in conjunction with the Family Wand to work. The book was given to Justin by Jerry along with the Family Wand, until Alex stole the book. When it's created an alternate reality where Jerry and Theresa never met, the book happens to be in the hands of Jerry.