Many Witches, Wiccans, and occultists work with trance and altered states of consciousness as a major part of their practice. If you are interested in getting started in this sort of work check out the introductory books listed in Pagan Bookshelf: Trance. If you’re ready for more here are some suggestions to get you even more involved.
“Travelling Between the Worlds: Conversations with Contemporary Shamans” by Hillary S. Web. This book is a collections of interviews with practitioners discussing trance and altered states in their spiritual practice.
“Between the Gates: Lucid Dreaming, Astral Projection, and the Body of Light in Western Esotericism” by Mark Stavish.
“Portals: Opening Doorways to Other Realities Through the Senses” by Lynne Hume.
“Soul Flight: Astral Projection & the Magical Universe” by Donald Tyson.
“Astral Dynamics: A New Approach to Out-Of-Body Experience” by Robert Bruce.
“Magical Imagination: The Keys to Magic” and also “Magical Pathworking” by Nick Farrell.
“Cosmic Meditation” by Michael Bertiaux.
“Seidr: The Gate is Open” by Katie Gerrard. (Trance work written by a practitioner of Norse witchcraft.)
“To Fly by Night: The Craft of the Hedgewitch” by Veronica Cummer. (This book was written for traditional witches.)
“The Clovenstone Workings” by Robin Artisson. (Another book written for traditional witches which contains a lot of material about working with altered states.)
“Ecstasia: An Introduction to Trancendental Music and Dance” by Julia R. Zay. (Working with altered states through music and dance.)
“The Uses of Ecstasy: Ritual and Practical Mysticism in Wicca” by Samuel Wagar.
“Ecstatic Witchcraft: Magick, Philosophy & Trance in the Shamanic Craft” by Gede Parma. (This book was written by an initiate of the Anderson Feri tradition but is not restricted to that tradition’s teachings.)
“Advanced Lucid Dreaming: The Power of Supplements” by Thomas Yuschak. (Subtitled: “How to Induce High Level Lucid Dreams & Out of Body Experiences.”)
And finally the best manual I’ve encountered for working with dreams which also applies to altered-state experiences is “The Jungian-Senoi Dreamwork Manual” by Strephon Kaplan-Williams. Lots of excellent material for processing and understanding these experiences.