The question of the People of the Book (Ahl al-Kitāb) has long been of central significance to Islam and later to the study of comparative religion. It becomes more crucial when Abū al-Fath Muhammad ‘Abd al-Karīm al-Shahrastānī (d.548/1153) develops another category known as “Man Lahu Shubhat Kitāb” (those who possess doubtful sacred scrolls) or “Ahl Shubhat Kitāb” (the People of a Dubious Book). The present book thus is of importance as it highlights the nature of the People of the Book and the People of a Dubious Book as understood in the Qur’ān and Islamic religious tradition. The discussion is carried out both by analyzing Islam as the only genuinely revealed religion as well as by addressing various Muslim scholars’ attitudes and responses to the reality and nature of the People of the Book and the People of a Dubious Book apropos the Qur’ān and the Islamic religious tradition. Evaluation of the nature of the People of a Dubious Book as exclusively developed by al-Shahrastānī is also discussed. It points out how he identifies these groups and how he treats them accordingly in his magnum opus, al-Milal wa al-Nihal. It is hoped that this book will provide a preliminary yet clear understanding of this concept, which hopefully can deepen our knowledge and enhance our perspective on the various delineations of the People of the Book and the People of a Dubious Book employed by Muslim scholars. This will eventually widen our sphere of the understanding of religious issues and any misunderstanding or narrowed perception of other religious traditions could be avoided.