An interesting extended essay on the relationship between pyschedelics and the spiritual or mystic, informed by Huxley's first hand experience of mescaline use. He documents in detail the progression of his experiences, the fascination with pattern and detail and bending of time and emotion, and used this to explain his perspective on how psychedelics could be used to enhance one's insight.

I haven't tried psychedelics but this does relate to something else that I experienced, so it's interesting to see Huxley's perspective on this and his active promotion of mescaline as a tool in this way. Obviously this book is old as hell now (and sometimes that is quite plainly apparent) so it's an avenue for which it's probably worth looking into more modern literature, esp as modern substances such as LSD etc weren't even created until decades later.