This study examines the reading passages in Vietnamese upper secondary English textbooks across three curriculum revisions (2006, 2012, and 2018). A corpus analysis was conducted to quantitatively analyze lexical, cohesion, syntactic, and readability features with Coh-Metrix. The 2006 edition exhibited a clear progression in complexity, with significant differences in word count, sentence length, lexical diversity and readability across grade levels. The 2012 and 2018 editions, while generally more complex, showed less consistent patterns of increasing difficulty. While these revisions suggest an overall trend towards more challenging texts, the lack of consistent patterns in other features highlights the need for a deeper understanding of how textbook features interact to influence reading difficulty. Future research should explore the qualitative aspects of textbook content to provide insights into how specific lexical choices and syntactic structures impact comprehension.