Ask HN: Where to Find Obscure Literature?

While reading about William S. Burroughs' "Cities of the Red Night" I found out about Konstantīns Raudive's book "Breaktrhough".

This book was about electronic voice phenomena (EVP) and the attempt at recording voices from the afterlife. Even if you don't buy into the concept of it, the content struck me as highly poetic.

I was fascinated by the book (which I never heard of) and was wondering if anyone could recommended me resources to find more "obscure", "interesting" and completely "left field" literature. (I put the previous words in quotations because these are things which are hard to quantify).