Phantastes by George MacDonald

According to Wikipedia: "George MacDonald (10 December 1824 – 18 September 1905) was a Scottish author, poet, and Christian minister. Known particularly for his poignant fairy tales and fantasy novels, George MacDonald inspired many authors, such as W. H. Auden, J. R. R. Tolkien, C. S. Lewis, E. Nesbit and Madeleine L'Engle. It was C.S. Lewis that wrote that he regarded MacDonald as his "master": "Picking up a copy of Phantastes one day at a train-station bookstall, I began to read. A few hours later," said Lewis, "I knew that I had crossed a great frontier." G. K. Chesterton cited The Princess and the Goblin as a book that had "made a difference to my whole existence.""