This book provides a systematic, empirical account of the language typically presented in English as a Foreign Language (EFL) textbooks, based on a large corpus of EFL textbooks used in secondary schools. A modified version of the Multi-Dimensional Analysis (MDA) framework serves to examine linguistic variation both within textbooks and compared to corpora representing ‘real-life’ English as used outside the EFL classroom. The results highlight the characteristics of Textbook English that define it as a distinct variety of English. In light of the study's pedagogical implications, this book proposes a range of corpus-based approaches to improve the naturalness of textbook texts. It also contributes to advancing quantitative corpus linguistics methodology: its detailed online supplements aim for methodological transparency and reproducibility in line with the principles of Open Science. This book will be of interest to linguistics and language education students and researchers, as well as EFL teachers, textbook authors and editors, and those involved in curriculum development and teacher training.