Lilith

This is the story of Mr Vane, an orphan and heir to a large house - in which he has a vision that leads him through an old mirror into another world. In the trips he makes to this other world, he explores the mystery of evil. Mr. Vane owns a library that seems to be haunted by the former librarian, who looks much like a raven. After finally encountering the mysterious Mr. Raven, Vane learns that Raven had known his father; indeed, Vane's father had visited the strange parallel universe from which Raven comes and goes. Vane follows Raven into the world through the mirror.

Lilith, written by the father of fantasy literature, George MacDonald, is considered among the darkest of MacDonald's works, and among the most profound.
George MacDonald is one of the most severely underrated authors of all time. A contemporary to Lewis Carroll and major influence on C.S. Lewis and J.R.R. Tolkien, this man's imagination apparently knew no bounds and that is incredibly apparent in his masterpiece, Lilith.