This article examines how the language of science and non-science texts differred across levels in a book series which is used in teaching English as a foreign language (TEFL). Employing Systemic Functional Linguistics (SFL) as the principal theoretical and analytical framework, this research examines linguistic features characterizing complexity, namely lexical density and nominalization of 24 reading texts in both science and non-science fields. The result shows that while the language grew more complex as the book levels advanced, the linguistic features of the scienceoriented and non-science oriented texts were not significantly different in the same book level. Based on a discussion of the findings, this article suggests that English textbooks should include texts that use genre and field-appropriate language in order to help students acquire technical and specialised language to prepare them for success in higher education and the workplace.