Fifty percent of IB schools in Japan use both the national and IB-authorized English textbooks. This chapter will explore the issue of how these differ. Books used for the study came from MYP Phase 3, MYP Phase 4, DP English B, and each grade of Japanese high school. In Study 1, 150 texts were checked for various language features: length, vocabulary profile, and readability. Text length was well-controlled in the Japanese textbooks, while IB books have both short and long texts. For vocabulary, AWL words appeared most often as the grade goes up. Analysis of English Vocabulary Profile showed that fewer CEFR A1/A2 words are used and more CEFR B/C level words appear as the grade goes up. Readability indices also show a wide range of levels. In Study 2, reading tasks are categorized in two ways: pre-/while-/post-reading tasks, and Anderson's taxonomy. In the IB textbooks, about 50% of the reading tasks require higher-order thinking skills whereas 70% of Japanese textbooks have tasks only requiring low-order thinking skills. This shows the sharp contrast between the two types.