Recommendations for interesting books about hidden dimensions of consciousness, perception, or mental transcendence?

I finished a short fictional story called 'Through The Gates Of The Silver Key'. I found the journey through new universes and vistas of reality intriguing.

So far, I've found 'Mysticism: A Study In The Nature And Development Of Man's Spiritual Consciousness' by Evelyn Underhill and 'The Doors Of Perception' by Aldous Huxley.

Are there any spiritual or philosophical works that explore such topics more deeply?

Any period is fine. Also note that I like rich wording or extravagant/purple prose.

I'm not certain if you're looking for nonfiction or fiction.

>2 SandraArdnas: Either is fine. :) I'm open to any recommendations.