This book is an English translation of the award-winning book by Mohammad Ali Amir-Moezzi on the Book of the Proof (Kitab al-Hujja) part of the Sufficient Book (Kitab al-Kafi), authored by one of the most celebrated religious authorities and a foundational figure of Shi'i Islam, Muhammad ibn Ya'qub al-Kulayni (d. ca. 940 CE). The significance of this source for the formation and development of Shi'i beliefs and doctrines cannot be overestimated. Whilst the field of Shi'i studies is still dominated by works focused on legal, political, or theological aspects of Sh'i Islam, Amir-Moezzi's approach to the study of the early Shi'i traditions in this work is characterised by a keen interest in its esoteric, mystical aspects. Complemented by detailed analytical comments, Amir-Moezzi's book demonstrates how al-Kulayni was able to set out an extraordinarily wide-ranging and coherent set of doctrinal and legal traditions derived from the Imams, thus playing a leading role in the unification and, consequently, the consolidation of the Imami faith. More specifically, by elaborating on the nature of the 'Proof', which in Shi'i terminology identifies the central figure, i.e., the guide (imam) of mystical spirituality, Amir-Moezzi's study portrays Shi'i Islam as multifaceted, not merely as the politico-religious ideology of its 'clergy', but above all as an inspirational religion. As Amir-Moezzi himself puts it, 'enriching Muslim thought and spirituality, sometimes with unusual finesse and sophistication'.