In EFL countries where English is rarely practiced outside the classroom, textbooks have become the major input source for learners. Particularly in Vietnam, multiple textbook series are available simultaneously for the same grade. Thus, it is important to examine if their vocabulary is appropriate and of similar difficulty. This study aims to…

Vocabulary knowledge is central to the process of reading comprehension (Cromely & Azevedo, 2007; Stahl & Nagy, 2005; Stanovich, 1986). The majority of our vocabulary knowledge is postulated to come from the process of incidental vocabulary acquisition (IVA) while reading (Nagy & Anderson, 1984). Prior studies have estimated an average…