"By setting out the philosophical part of spiritual science in the “Book of Spirits,” we offer here the practical part for those who wish to engage in these phenomena themselves or want to be able to give an account of the phenomena they will witness. This book will point out the dangers that may be encountered, and thus provide a means of avoiding them. These two books, although the last serves as a continuation of the first, are to some extent independent of each other; but for those who wish to seriously study this subject, we advise first reading the “Book of Spirits,” because it contains the basic principles without which some parts of the “Book of Mediums” may be difficult to understand" (A. Kardec).