There's an interesting account of how Colin Wilson came to write "The Occult" in chapter 18 of his autobiographical work "Dreaming to Some Purpose" (2011). It's a fairly generous preview, but some pages are missing. In chapter 19 he covers the sequel, "Mysteries", but for me the text stopped just after he started to talk about the book. If anyone is curious to read the whole thing, the book is available for £2.49:
https://books.google.com/books?id=ZCvrS9DjE68C
https://books.google.com/books?id=ZCvrS9DjE68C