by Arthur Machen
Mary, a young woman in Wales, has her mind destroyed by Dr Raymond’s attempt to enable her to see the god of nature, Pan. Years later the beautiful but sinister-looking Helen Vaughan arrives on the London social scene, disturbing many young men and causing some of them to commit suicide. - from wikipedia
The link above is to project Gutenberg, where you may download the novella freely, in several device formats. It is also available directly from Amazon in Kindle format at no cost.