The rapid advancement of electronic textbooks (e-textbooks) has reshaped digital learning, yet the literature on their classification remains highly fragmented. Existing models categorize e-textbooks based on technological sophistication and interactivity, but they often lack an evolutionary perspective, making it difficult to assess how these resources have progressed over time. This paper aims to address this gap by proposing a comprehensive, evolutionary taxonomy that classifies e-textbooks into five generations: Digitized, Rich, Interactive, Immersive, and Intelligent e-textbooks. Each category is analyzed in terms of technological enablers, pedagogical implications, and limitations. Digitized e-textbooks represent the earliest form, offering static digital replicas of printed books with minimal functionality. The Rich e-textbook generation introduced multimedia elements, enhancing engagement but lacking true interactivity. Interactive e-textbooks advanced learning by incorporating self-assessments, quizzes, and feedback mechanisms, supporting active learning and formative assessment. The Immersive e-textbook generation, leveraging Augmented Reality (AR), Virtual Reality (VR), Mixed Reality (MR) and "phygital" learning, provided spatial and experiential learning environments. Finally, Intelligent e-textbooks, powered by Artificial Intelligence (AI), Natural Language Processing (NLP), and adaptive learning systems, enable personalized, data-driven educational experiences, dynamically adapting to individual learners. Despite these advancements, significant challenges remain. The lack of standardization, high development costs, cognitive overload risks, privacy concerns in AI-driven learning, and limited digital literacy hinder widespread adoption. However, emerging opportunities in AI-driven adaptive learning, blockchain-based credentialing, phygital textbook integration, and the expansion of Open Educational Resources (OER) present avenues for future research and development. By consolidating fragmented research into a structured, evolutionary framework, this paper provides a clearer understanding of e-textbooks' trajectory and highlights key considerations for the next generation of digital textbooks. As e-textbooks continue to evolve, the integration of Extended Reality (XR), AI-generated content, and blockchain-secured academic credentials will further redefine personalized and immersive education. The future of e-textbooks lies in balancing technological advancements with pedagogical, ethical, and accessibility considerations, ensuring that digital learning remains inclusive, engaging, and effective.

Keywords: Electronic Textbooks, Educational Technology, Taxonomy.