Abstract: This thesis examines the function of the illustrated Buddhist books of South Asia as sacred objects and how their 'sacredness' is created, made to operate, intensified, and maintained. It also explains how the supposedly unrelated text and images in fact relate to each other and what this relationship could tell us about the characteristics of Buddhism as practiced in India in its last stage and its regional development in Nepal. Buddhist books of South Asia that record the Buddha's supposed words and teachings on palm-leaves and paper had been so copiously and artfully produced during the eleventh and twelfth centuries in South Asia that we can propose a book-cult in which the book is not only a text but also a sacred object that links transcendent spirituality to the ephemeral world. This book-cult has survived till the present day in Nepal while evolving around the local cultural context of different periods, reflecting the various needs of the local communities. However, Buddhist books of South Asia have never been fully studied as cult objects; instead, they have mainly been understood as texts. Even when art historically approached, only the illustrations are individually discussed as if viewed with a super zoom lens and the materiality of a book is completely forgotten, let alone the relationship between the text and the images in the book. To determine the mechanism of the 'sacredness' of the book, I have examined the Buddhist books of South Asia from utilitarian and material perspectives with archaeological modes of analysis, while locating them in the cultural atmosphere of the time and region. After a thorough study of the iconographic programs with the close reading of the manuscript text, I argue that the images have a metonymical relationship to the text and they collectively serve as an icon of the book while intensifying the sacrality of the book. I also argue that the changes in iconographic choices in manuscript illustrations reflect the changes of interpretation of the older doctrine and practice in cultic-Mahāyāna and Vajrayāna schools at the last stage of Buddhist development in India. The first two chapters explore the history of illustrated manuscripts in eastern India. Chapters 3 and 4 investigate the iconographic programs in the illustrated manuscripts and their significance in understanding the Buddhist culture of the time. Chapter 5 examines the function and the use of the book as a cultic object by exploring the representations of the book as well as the physical evidence of the book cult recorded on the body of the book. In the after word, I provide a brief account of the book-cult as it survives in Nepal, which provides a window through which to understand the book-cult of the past.