I can understand why he's not widely read. He's difficult, truly challenging, strangely exhausting. I still don't have my head wrapped around the Golden Key, and I'm sure I'm still missing things about The Day Boy and The Night Girl, which review follows.
Two things strike me most about MacDonald, on the surface. First is his insight into his characters, especially his villains. You can tell that he's known, and loved, people just like all the most wicked characters he writes. Second is his science-fiction sensibility. More on that.
The Day Boy and the Night Girl is a very short novel, three hours on the LibriVox recording I listened to (narrated by the delightful Laurie Ann Walden). On its face it's about a kidnapping witch who raises two children, one only seeing sunlight and the other raised in perpetual gloom. MacDonald's not the type to put a stock fairy-tale witch in his story. Here's his description of the witch and her motivation:
"THERE was once a witch who desired to know everything. But the wiser a witch is, the harder she knocks her head against the wall when she comes to it. Her name was Watho, and she had a wolf in her mind. She cared for nothing in itself -- only for knowing it. She was not naturally cruel, but the wolf had made her cruel."
In other words, Watho is a mad scientist. She steals the children to conduct experiments on them, very scientifically if in fairy-tale mode; for example, the day boy's mother is fed on venison while the night girl's mother is given pomegranates.
The experiment works, and the children take on the aspects of their respective elements. Photogen takes on the aspect of the day, strong and brave and happy, while Nycteris takes on night-attributes of contemplation, quietude, patience.
Much more of the prose is devoted to Nycteris, and her discovery of the night, which she takes to be the day, is beautifully described as a poetic scientific exploration.
"She knew nothing of water but from what she drank and what she bathed in; and as the moon shone on the dark, swift stream, singing lustily as it flowed, she did not doubt the river was alive, a swift rushing serpent of life, going -- out? -- whither? And then she wondered if what was brought into her rooms had been killed that she might drink it, and have her bath in it. "
Every new thing she learns, deluded as she is, brings her joy. The witch, meanwhile, with all the resources and knowledge she has, and this wildly successful experiment, is terribly unhappy.
"Also, Watho had a poor, helpless, rudimentary spleen of a conscience left, just enough to make her uncomfortable, and therefore more wicked."
So there's a cautionary tale about the way we conduct science from 1882. That's only a subplot, though, as Photogen eventually discovers the night and is overwhelmed with fear, and as he and Nycteris discover they can only rely on each other when they're out of their element.
There's a spiritual parable in that, as there always is when MacDonald is involved. Nycteris starts with nothing. Her world is the size of her room. Every thing outside of that room is new and glorious, until the glory of the sun becomes too much. Photogen starts with everything, constantly experiencing that glory, and has become unable to handle anything less.
Although I wouldn't say MacDonald intends the sun to represent the presence of the Spirit. The night is presented as a necessary place of peace and calm, a necessary counterbalance to the day, not an equal force - for how could there be a comparison? - but a like one. Just as the day and night are both necessary, so are the masculine and feminine, and Photogen and Nycteris embody those aspects as well. It's a wonderful story of balance, from the Christian perspective where balanced things go places.