The Hill of Dreams is a great recommendation, it doesn't get mentioned often enough (or at all :-) I think. I think his The Three Impostors is even better.
The early 1900s mystical fiction is a rabbit hole for sure. Voyage to Arcturus is a very good recommendation! Others in this vein include, but are not limited to:
If you like this sort of weird early 1900's mystic book, be sure to also check out David Lindsay's Voyage to Arcturus (https://en.wikipedia.org/wiki/A_Voyage_to_Arcturus).