One of the first fully preserved act books is published in full - the Lutsk Castle Book for December 1560 and the entire 1561, numbering 383 records of judicial and notarial cases - a fundamental historical source on the history of Volhynia, which was part of the Grand Duchy of Lithuania, Russian and Samogitian. This is the first book formed for the management of Prince Bogush Fedorovich Koretsky as a Lutsk headman. The castle book is a product of the powers and reference work of a judicial and administrative institution (castle government) in the capital of Volyn land. The case materials reflect the social structure and dynamics of the development of class society in Volyn on the eve of the Union of Lublin in 1569 - princes, gentlemen, gentry, boyars, servants, peasants. reflects economic, property and conflict relations between gentry estates, officials (officers), Lutsk bourgeois, merchants. The scope of legal proceedings is exhaustively represented. The text of the monument is a unique source of study of the old Ukrainian written and literary language in its northern Ukrainian version. Many linguistic and historical facts are attested for the first time. Transliterated text accompanies historical, linguistic-paleographic research, pointers of words and word forms, nominal and geographical.