I'd give you a different answer at different times but I'd say this is the best sci-fi novel you haven't read

https://en.wikipedia.org/wiki/Worlds_(novel)

though I'd say

https://en.wikipedia.org/wiki/Operation_Chaos_(novel)

is the best fantasy novel you've never read.

The Initiate: Some Impressions of a Great Soul (1920) by Cyril Scott

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