This book contains 419 letters from the era of Buyid dominance over the Abbasid Caliphate and their control over the state. The significance of these letters lies in preserving much of the primary material assisting in studying the history of the Islamic state during the Buyid rule (334-447 AH/946-1055 AD). These letters reveal political correspondences, economic arrangements, and administrative procedures. They include documents, covenants, and appointments to major positions. Readers will find a treasure trove of valuable benefits, providing researchers with a wealth of new historical material published for the first time. It is rich in raw materials that bolster historical research and solidify its foundations. The content largely comprises official letters issued either by Abbasid caliphs or Buyid princes, who managed to appropriate many eastern Islamic lands, extract them from the Caliphate's authority, and undermine the Abbasid caliph's worldly powers to control the state's resources. These letters showcase their view of the state, their administrative philosophy, and their economic, administrative, political, military, and architectural measures, as well as their relationships with caliphs, people, and each other, revealing much of their social and cultural history. The collection also includes personal letters covering congratulations, condolences, thanks, reproaches, intercessions, and appeals, reflecting a network of social relationships, regardless of the sincerity of their emotions and feelings.