I subscribe to Scribd which is $9 a month and has thousands of books on Shamanism and spirituality. They even give you a free three month trial.
I personally recommend the following books that can be found on their app:
The Way Of The Shaman - by Michael Harner
Shamanism : The Book Of Journeys by Robert Levy and Eve Bruce
The World of Shamanism: New Views of an Ancient Tradition by Roger Walsh
Soul Retrieval: Mending the Fragmented Self by Sandra Ingerman
The Evolutionary Mind: Conversations on Science, Imagination & Spirit by Terrence McKenna, Rupert Sheldrake and Ralph Abraham
Visionary Plant Consciousness: The Shamanic Teachings of the Plant World by Park Street Press
The Ayahuasca Experience: A Sourcebook on the Sacred Vine of Spirits by Park Street Press
Plant Spirit Shamanism: Traditional Techniques for Healing the Soul by Ross Heaven, Howard G Charing and Pablo Amaringo.