The purpose of this dissertation is to investigate linguistic variation within ESL/EFL reading textbooks and between EAP reading textbooks and lower-division university textbooks. Specifically, within ESL/EFL reading textbooks, lexical, lexico-grammatical, and formulaic variation was measured within series of textbooks, across textbooks at the same proficiency level, across the various proficiency levels, and across text types. The research of this study was based on a corpus of 563 ESL/EFL reading textbook passages representing five complete series and five proficiency levels. Additionally, a subcorpus containing high-level ESL/EFL reading textbooks represented EAP reading textbooks, which was compared to a corpus representing university textbooks used in the first two years of university containing text excerpts from a variety of disciplines. To assess lexical variation, I calculated the normed number of word types per textbook, high frequency vocabulary coverage, and general academic vocabulary coverage. An additional keyword analysis was conducted to identify salient semantic themes. The lexico-grammatical analysis consisted of mapping the corpora onto the first three dimensions from Biber's (2006) lexico-grammatical study of university language. Formulaic variation was assessed by identifying lexical bundles and comparing structural and functional distributions across levels. I found that in most cases, textbooks series did not have complete incremental progression, meaning textbooks did not become more linguistically from book to book within a series although there was generally progression from the lowest to the highest textbook. Additionally, intra-level inconsistency was observed as books of the same level varied linguistically. An interaction effect was observed between textbook publisher and proficiency level as it appeared that each publisher adhered to a different standard for leveling books. Text type was also found to be an important factor, which interacted with proficiency level. That is, informational texts tended to be more linguistically challenging than non-informational texts, and informational texts progressed across proficiency levels whereas non-informational texts tended to be more linguistically homogeneous and displayed less variation across levels. Moreover, considerable linguistic gaps were observed between EAP reading textbooks and lower-division university textbooks with EAP reading textbooks being less linguistically complex. These findings can be used to inform educators in their textbook selection process as they strive to match learners with appropriate texts. Furthermore, materials designers can use the findings to inform future textbook iterations. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]