Yatvigian Book (hereinafter YB) is a conventional, probably the most exhaustive and the most important description of the ethno-cultural tradition of the tribe that spoke Yatvigian, one of the two languages of Western Balts, recorded during the Reformation period. It is based on the source of information disseminated in several variants of manuscripts, and later in small printed books (reprints). Unfortunately, the original of YB has not been found yet, and the scholars who discussed this source or analysed it to some detail after the appearance of Wilhelm Mannhardt’s book Letto-Preussische Götterlehre (1936), which contains copy A(p), resorted not to the analysis of the copies, but to the materials provided in the book, and did not doubt the authenticity of the given information. Due to these reasons, historical facts of the manuscripts were not investigated as the information in Mannhardt’s monograph was taken for granted. Unfortunately, in many cases it does not correspond to reality and is essentially erroneous or elliptical. Textological analysis of ten surviving manuscripts of YB shows that the method of information structuring in the copies A(p), α, B, X of the old edition presupposes a reflection of the mandatory tradition of the preparation of diplomatic documents, i.e., those that are related to the legal field: the presentation of factual material follows strict rules established for preparation of such works – a prologue, a narrative, and an epilogue. Although this circumstance has not been highlighted by any of the researchers of the source analysed, it is extremely important, because it can be related with the intent of the work and the aim
of its creation. Starting with copy C, whose author doctor Gregor Duncker decided to remove the foreword of YB and to retell the remaining narrative instead of copying it, a precedent was created for evaluating this work not as a supposedly important document of the juridical field, but as a work of prose. Unfortunately, the creator of manuscript K distorted the information of this source even to a greater extent, removed the information on the lexicon used by the Yatvigians that he deemed unnecessary, and transformed this important work into something like a rough copy.