New Theory of Numerology

"Thanks for the book, Harry! I've been wanting that New Theory of Numerology for ages! And that perfume is really unusual, Ron."

- Hermione Granger to Harry Potter and Ron Weasley

New Theory of Numerology is a book that seems to be about Arithmancy, but is not a required text for the subject at Hogwarts School of Witchcraft and Wizardry. Harry Potter gave a copy to Hermione Granger for Christmas in 1995.