I am searching for a series of young adult books I read approximately 10 years ago (2010). They were from a library so I assume they can't have come out before 1990.
There are two protagonists: a young girl of around 13-16 and a wizard who is much older (50+).
As a consequence of a ritual performed at the beginning of the first book, the wizard looks permanently in his mid-twenties despite being much older. I think he binds his soul to a book, he may also be missing an arm.
At some point, the girl uses the book to enact another ritual which ages her about 10 years. She then attempts to romantically pursue the wizard. The ritual makes use of a lot of blood.
There is a villain who wears the pelt of a wolf and is able to shapeshift into a wolf. In one of the later books, he wears the skin of a person who can shapeshift into anything and gains this power.