I'm sure my reading was affected by having already read some about it in The Christian Goddess: Archetype and Theology in the Fantasies of George MacDonald. So I came at it from outside, a bit, and yet.... it remains a compelling story, driving forward and making me long to know what might happen next.
The imagery and theology is dense and layered and frequently subtle, unless you are really paying attention. This is the kind of book that I could spend months on, doing a close reading just of the light-and-dark symbolism, or of the names, and fill many pages with what I found.
Some of the themes hit close to home, despite the fact that I am 36 and supposedly well past this sort of “coming of age” tale. (The protagonist is quite a young man.) The theme I'm carrying with me at the moment is the idea that in order to get anywhere, you first “must make yourself at home.” If you want to see how MacDonald works out this idea, I'm going to have to refer you to the book.
Many readers of C.S. Lewis are aware of the influence MacDonald had on him, especially the iconic acknowledgment that Phantastes “baptized... [Lewis'] imagination” when he was still a young atheist. I've never heard of Lewis mentioning Lilith, but he must have read this book multiple times; there are images in it that I first encountered in Lewis' work and have rarely or never seen elsewhere. For example: the living, growing stones in the Underworld (The Silver Chair), going further up and further in (The Last Battle), water at the end of the world (The Dawn Treader) and going into another country and returning (although that is generic to this sort of hero's journey fantasy).
I'd recommend this book to any fan of Lewis or MacDonald, but also to anyone who is intrigued by symbolism and by the psychological intensities of the hero's journey form. Even if you're annoyed by Christianity in general, I'd still say you should give this a try—it's the best of Christian theology, distilled in the story of a developing human soul.