Scholars have studied the sources and organization of pillar works of the Islamic tradition such as al-Isfahani's Book of Songs (Kilpatrick, 2003), al-Tabari's History of Prophet and Kings (several translators, 1980s), and Mas'udi's The Meadows of Gold (Khalidi, 1975). However, the field of medieval Islamic history lacks any studies tracing the history of such books. Just the fact that al-Tabari wrote a large work of history does not ensure its survival to the next generation, let alone later subsequent generations. What dynamics do bring certain books to the forefront while pushing others into obscurity or complete oblivion? How can we study such dynamics? This study traces the journey of a ninth-century biographical dictionary to Muhammad ibn Sa'd (d. 230/845), Kitab al-Tabaqat al-Kabir (KTK), from near obscurity to canonical status. By tracking borrowings from the KTK in later biographical dictionaries, the study confirms that many, but minor, additions were made to the book after its author's death. It also uncovers that the extant manuscripts of the KTK are based on a recension of the work that was obscure until the fifth century. Starting in the fifth/eleventh century and continuing in the sixth/twelfth and seventh/thirteenth centuries, the center of diffusion of the KTK moved slowly from Baghdad to Damascus and Cairo. In these new homes, the book became one of the most authoritative sources about early Islamic figures, especially those who participated in the transmission of the Prophetic tradition, Hadith. The study also exposes the two contradictory trends working to shape and reshape Islamic historiography, book fragmentation and book transmission. The first trend decomposes works in order to form new ones. Whereas the second trend ensures the survival of some chosen books and imposes them as "canonical" works. Book fragmentation was done by enterprising and ambitious compilers seeking to build a name and a legacy for themselves; whereas book transmission was done by second-tier Hadith transmitters, who dedicated their time to teaching. Transmitters of the KTK first followed the strict rules of dictation (sama'), while approving small parts of the work copied without dictation. By the ninth/sixteenth century, the written or oral approval of a certified owner was sufficient to transform a copy of the KTK into a certified replica of the original. The KTK has thus reached the canonical status, where alterations did not occur because they would simply be immediately detected.