Shamanism is a primal path, probably the oldest form of religious, mystical and magical expression known to Homo sapiens, and a parallel and overlapping stream with that of Witchcraft. The word shaman comes from Siberia, and properly implies a male practitioner of that region (the female equivalent being a shamanka). However, the term is now used to describe similar practitioners, of either gender, in traditional societies across the world. Furthermore, it has been taken up by Western people seeking and learning a more primal path. However, there are some genuinely shamanic people in the West, who really have undergone a shamanic initiation through near-death experience and being chosen by the Gods, spirits or however the unseen powers are perceived. There are also many who profess shamanism who have adopted the outer forms without the inner changes. Tread carefully in the New Age fairs!
The following books may be useful: