Shaman is not a general term that happens to be applied to Siberia. It is a Tungus word, saman, which entered Russian from Evenki speakers in the seventeenth century, passed into German and then into every other European language, and was gradually stretched to cover ritual specialists on six continents. Shirokogorov is the ethnographer of the people it came from.
He worked among Tungus groups in Transbaikalia and in Manchuria between 1912 and 1917 for the Russian Academy of Sciences, in long stays rather than short visits, and he learned the language. Social Organization of the Northern Tungus appeared in 1929 and Psychomental Complex of the Tungus in 1935, both published in China, where he had settled after leaving Russia.
The second book is the one that matters here. It is an attempt to describe an entire mental and spiritual system, spirits, souls, illness, dreams, the practitioner and the community, as one working arrangement rather than as a list of beliefs held. It runs to several hundred pages, it is unusually specific about what Tungus people themselves said and how they said it, and it remains the fullest single account of any Siberian religious system in a European language. It is also, because of where and when it was printed, one of the harder classics of the field to find.