It is the second book on the Codification of Spiritism, having been published in January 1861. After the publication of The Spirits' Book, in 1857, Kardec launched, in 58, a little book entitled Practical Instructions on Spiritist Manifestations, which was an essay for the Mediums' Book, having suppressed the one when it published this. The Mediums 'Book brings together the Spirits' special teaching on the theory of all kinds of manifestations, the means of communication with the invisible world, the development of mediumship, the difficulties and obstacles that can be found in the practice of Spiritism, constituting the follow-up to The Spirits' Book. In the introductory part of the work, Kardec explains what the work is intended for, elucidating that daily experience brings confirmation that the difficulties and disillusions existing in the practice of Spiritism originate from ignorance of its principles, being satisfied in proving that its work produced good fruit. It is natural to know that among those concerned with Spiritism, there is a desire for communication with the Spirits, serving the work exactly to guide this path, taking advantage of some of the studies carried out by the Codifier, as it is not enough to place fingers on a table , in order to make it move, or hold a pencil, in order to write. He also adds that he is mistaken who supposes to find in the work a universal and infallible recipe to form mediums, since, while each one brings within him the germ of the qualities necessary to become a medium, the potentials exist in different degrees and their development depends causes that we cannot verify at will. He says that the objective of the work is to indicate the means of development of the mediumistic faculty, as far as the dispositions of each one allow, and, essentially, to lead to the useful use of mediumship, when it exists. We know that mediumistic manifestations continually grow in the whole world, not only within the spiritist activities, becoming more and more important to understand the phenomena and their peculiarities. The Mediums' Book, although it was written over a hundred years ago, is extremely up-to-date, and there is no other work that has surpassed it today, being certainly a treatise based on scientific research and experience, in addition to the valuable theoretical contribution of the Spirits in the explanation of several problems that are still inaccessible to scientific research, which is why the book is recommended to everyone who is willing to communicate with the Spiritual World.