This study aims to establish a link between a previously-developed Book Index based on academic vocabulary and students’Reading Index. To achieve this, the students’ reading abilities were divided into two main components: reading comprehension and vocabulary. Assessment items were developed for each subcomponent and organized into a five-level test form. A preliminary survey was conducted with 1,626 elementary, middle, and high school students. Based on the expected accuracy rates corresponding to item difficulty, a linear equating method was applied to calculate each student’s Reading Index. This index was then linked the Book Index to facilitate reading activities appropriate to each student’s individual level. The significance of this study lies in its development of a stepwise, adaptive assessment model for evaluating students’ Reading Index. It also proposes a method to connect assessment outcomes with the Book Index.