A Romance of Two Worlds by Maria Corelli provides you with so much to think about, your mind will be racing for the duration of each chapter. The obvious thing you'll be thinking about is the story as presented: a musician goes abroad in an attempt to rest and recover from depression and illness. There, she meets a mystical doctor who claims he can heal her physically and spiritually. This begins a journey for her in many different senses of the word, and it all reads like a cross between really good sci fi and crackpot religion.
But it's a fascinating book, ending in a sort of philosophical and theological climax in the same way that, say, Stranger in a Strange Land or 2001 do. There's a lot of metaphysics in the book, which lends for some fascinating speculation, from a gamer's perspective especially, about how magic systems in a good RPG might be explained, but it's blended expertly with good old-fashioned (literally!) technobabble and pseudo-science. What could be more science fiction than that?
The plot has a touch of mystery and a smattering of character drama, but around all of that hovers an obsessive existential angst. The author clearly put a lot of thought into the philosophy of her story, and has her own unique reinterpretation of Christianity, which she attributes to the fictional doctor in the book. It's so authentic, you have to wonder about the appendix, containing letters to the author from fans who thank her for, basically, enlightenment. It could be the old "based on a true story!" trick, but you never can tell.
And still there's more going on in the book. It was written during a time when electricity had just come into semi-regular use, after the trans-Atlantic telegraph cable had been run. You can sense the excitement and optimism in the fiction: electricity is a mystical force of the world that connects the physical plane with the spiritual one, and science puts everything past and present into perspective. The author's fascination for this up-and-coming technology is both infectious and uncomfortable, because you want her to be right about there being a bright future for humanity, but you are a reader literally from her future, so you know how it actually turns out.
The Victorian setting makes the story a sociologically interesting, too. Getting the world view from a female Victorian artist is fascinating. It's enthralling to read about how she sees her place in the world, and the way she feels other sees her and her gender, and what she wants to achieve, and what she wants to do with her life, and her insecurities and doubts, and the strength she finds within herself. It's honestly one of the most feminist stories I've ever read, and it's progressive feminism: the kind that doesn't want to mimic the world of men, but rejects it and seeks to reform it.
There's even more to discover in this book, but I can't mention everything without venturing into spoilers. If you like early SF, this is well worth a read, and available for free from gutenberg.org as well as Librivox.