Greetings! I wish to recommend "Taliesin: the Last Celtic Shaman" by John Matthews for reading. Though I have not read the whole book, I believe the shamanistic journey and meditation are the same, and the meditative state can be easily reached with out hallucinogenic substances. It is a matter of what you do when the meditative state is reached. In the traditional bardic schools mentioned by Ceasar, Strabo, and, maybe other classical writers, and the Marquis of Clanricarde, a bardic student is placed in a quiet environment to receive inspiration, the awen, to compose a poem or story. In a shamanistic journey, the shaman goes into a trance to travel the otherworld to fulfill the purpose of the shamanistic journey. I, also, recommend Michael Harner's "The Way of the Shaman" as a good begging book about shamanism.