This paper presents a novel methodology to determine textbook readability. The method mainly relies on students' lexical coverage to determine the readability level of textbooks, taking into consideration that readability depends primarily on vocabulary knowledge and that it is a function of lexical coverage in teaching English as a Foreign Language (EFL). The appropriateness of the reading material used is critical. If it is inappropriate, the chances that learners understand it are substantially jeopardized. This study examined seven EFL textbooks currently in use in Algerian public schools to determine whether learners' lexical coverage is at the textbook readability level (Independent Reading Level), above it (Instructional Reading Level), or below it (Frustrational Reading Level). It adopted a multi-instrument computer-based approach. Results showed that all the textbooks have low lexical coverage and readability, which puts them at the frustrational level. Moreover, it was found that 4 out of 7 textbooks demonstrated a discrepancy in terms of lexical coverage as the rate of common vocabulary across all textbooks was very low. Comparing the lexical coverage of the seven textbooks to standard vocabulary lists revealed that students are not learning sufficient, useful, and appropriate vocabulary.