Many projects around the world aim to use and develop digital textbooks. However, differences between aims, affordances, methodologies and technologies cause many issues, controversies and concerns to arise. This chapter aims to explore the basic structure of digital textbooks. The focus is on the non-linear mechanisms of eTextbooks and Open Source Textbooks. At the beginning is investigated the current state-of-art in order to arrange the distinctive features in a hierarchical manner. Then, it assembled the digital textbook infrastructure. The proposed methodology allows identifying the common elementary components of digital textbooks through meta-analyses of proposed methodologies. To arrive at this framework, it explored the diversity of contexts, educational platforms, PC, digital devices, contents and processes. The chapter concludes with a call for future research of digital textbooks functionalities, analysing them in a diversity of four learning environments.