I can't remember much about this book hence why I wanted to re-read it but its power cost is unique enough to set it apart.
People had this magic, very powerful but it had a hefty cost, any spell no matter how small turned a part of the casters body to stone. They could move regularly until they used it too much to the point they were 100% stone then they became a statue.
Another big thing I remember is the main character (a male kid) saved some girl from a basement and turns out she was some sort of entity. The girl in return gave the kid every spell created. So he knew how to do a bunch of spells but the cost was the same.