I’ve always found esoteric ideas interesting. I put “esoteric” into Google, and the first site that was returned (after dictionary definitions and Wikipedia) was a compilation of Western esoteric texts. It looks interesting.
And if you’d like the definition, here is what Merriam-Webster has to say:
b : requiring or exhibiting knowledge that is restricted to a small group <esoteric terminology>; broadly : difficult to understand <esoteric subjects>