As the difficulty level of EBS prep books designed for College Scholastic Ability Tests (CSAT) has recently been hotly debated, a new version of CSAT (Engish) has been proposed by the government to reflect the nature of criterion-referenced test. At this juncture, the present study is intended to investigate the extent to which the overall difficulty level of EBS-CSAT prep books appropriately reflects the National Curriculum. In order to achieve this goal, the study conducts a corpus-based analysis of the linguistic difficulty among High School English Textbooks, EBS-CSAT prep books and CSAT (collected from 2010 to 2013). Overall, the results revealed that the linguistic difficulty levels of EBS-CSAT prep books (which are similar to CSAT in many respects) are much higher than those of High School Textbooks in terms of vocabulary, syntactic complexity, coherence, and readability. In order to normalize the level of difficulty, EBS prep books are to be developed through more rigorous procedures that abide by the National Curriculum. Finally, employing an item-pool system based on textbooks is proposed as a viable approach to the aforementioned EBS-CSAT issue.