The ability to read is an important skill that must be possessed by a language learner. In the context of English Language Teaching (ELT), especially in formal schools, one of the important tools to hone reading skills is textbooks. Textbooks are a guide for teachers in teaching and an important resource of material for students. The textbooks contain texts designed to hone students' reading skills. Therefore, determining a textbook's readability is critical. This research used qualitative approach to investigated the text readability level of a High School English Textbook published by a private publisher from Indonesia by focusing on the reading texts. This study used Flesch Reading Ease, Flesch-Kincaid Grade Level, and Coh-Metrix L2 Readability formula. 26 passages selected from the English Textbook were analyzed automatically using Coh-Metrix. It was revealed that most of the reading texts were below the appropriate readability level of the targeted reader. It implies that English teachers are suggested to support the lack of suitable reading materials, the teacher is supposed to find texts from other sources to adjust the readability level of the students and achieve the successful acquisition of a second language.