Hermetic textbook language poses a significant problem in education of the young in our country. The goal of this paper is to point out to the complexity of vocabulary in textbooks, which contributes to students' lack of understanding of what they are reading. Since we established that textbooks in narrative subjects - history, geography and biology - are mostly used during studying and that students mark them as especially difficult for understanding, the subject of this research is precisely the analysis of words which seventh-grade students from primary school state as unfamiliar on a randomly selected, but balanced in terms of length, textbook text of those subjects. The results of frequency analysis indicate that there are a lot of unfamiliar expert words, and frequently the same number, or even more, of common Serbian words in textbook texts, especially in history. Approximately the same or even larger number of unfamiliar words occurs in familiar texts when compared to unfamiliar, which indicates that the previous usage of texts does not contribute to their understanding. Based on correlation analysis referring to the number of unfamiliar words, frequency of textbook usage, perception of difficulty of the text in textbook, general and the achievement in the particular subject, it was determined that unfamiliar words are not only mentioned by students with low grades, although they do it more often, nor that only these students are the ones complaining how difficult textbooks are to them. Based on regression analysis, the number of unfamiliar words, especially in the history textbook, even figures as a predictor of success, which seems contradictory and can be interpreted differently. The results point out to the fact that inaccessibility of textbook language mostly does not guide the students to learn new words, but causes revolt and thus disables the development of language, scientific concepts and acquiring professional terminology, which remains inaccessible even after having been studied in school.