Introduction. Traditionally, several books of the OT are ascribed to King Solomon, but, according to Jewish tradition, he wrote only three biblical books, viz.: Proverbs, Ecclesiastes and Song of Songs. According to early Christian, early Byzantine and medieval Armenian interpreters, this order of books in the Solomon trilogy is not accidental. Although in the Christian tradition, this order of books has been attested already from the first centuries, nevertheless, today there are discrepancies and not only about the order of the books in question. Each of these books brings forth a certain semantic and symbolic meaning. They are connected with each other, being, as they were, “steps of the stairs” on the way to knowledge, not to mention the fact that each of these books occupies its worthy place in the treasury of human thought. Methods and materials. These circumstances make the appeal to questions related to Solomon’s tripartite, which are worthy of research using a method of a comparative analysis of various sources. Such a conviction is supported by new data concerning the issues under discussion provided by Armenian sources. Among these testimonies аn important role will be assigned to the use of unpublished yet manuscripts on the topic. A special place is occupied by the codex, which contains an Armenian medieval isagogical collection of the late 12th – early 13th centuries. Analysis. It was compiled by the abbot of the Sanahin monastery Grigor, son of Abas, and is known under the provisional name Book of Causes. The full title of the book is attested only in one manuscript – the most important and earliest from all that have reached us, stored in the Mashtots’ Matenadaran under the number 1879. This medieval textbook includes isagogical questions or prolegomena (presented in Armenian manuscripts under different names, mainly “cause”, “beginning” – hence the Book of Causes) to the all canonical books of the Bible and to the so-called “subtle” writings. The “subtle writings” are certain works that include the writings of both “external” (in relation to Christianity) as well Christian (mainly – “church fathers”) authors. Being of a philosophical and religious character they serve sort of a connecting link, and not only between the OT and the NT. The “subtle writings” were irreplaceable when using philosophical ideas to prove or refute one or another dogmatic position or postulate. As far as can be judged today, this title of certain works has survived only in Armenian; there is no doubt, however, that it had its prototype, at least in the Greek tradition. From the above it follows that the Book of Causes is significant also by the fact that it contains mainly Armenian translations of the introductions to the Bible commentaries of the Church Fathers. Along with them, the Armenian original prolegomena to interpretations are presented here as well. In this manual, there are several prolegomena related to Solomon’s trilogy. Results. These chapters not only confirm the data of early Christian and early Byzantine authors, but also provide some interesting evidence that has not come down to us from other sources (as far as I know). I hope that suggested here analysis of Armenian sources, especially the testimonies from the unpublished isagogical textbook called the Book of Causes will contribute to the international knowledge concerning the discussed questions.