The book on mediums (o livros dos médiuns [Portuguese])

By Allan Kardec:

It is a descriptive book that does not teach to be a medium. This book is more boring than «the spirits' book», a book from the same author. Allan Kardec, before anything else, was a scientist and he respected the observational method and the rigor. However, the author was also a great writer in that he describes and explains impartially all the facts. I advise you to read «the spirits' book» first, and thus you already get a good perspective of what is Spiritualism, a branch of Christianity.