In the early 17th century, a Spanish fleet captured a French ship whose stolen cargo included the entire manuscript collection of the Sultan of Morocco, Muley Zidan. soon, the collection made its way to the royal library, El Escorial, transforming the library into an important repository of Arabic books which, since then, Arabists from across Europe sought to visit. By focusing on the social life of the collection, from the moment of its capture on the high seas up through the longue durée process of its incorporation into the Escorial, this paper examines three related issues: the first regards the social trajectories of books and the elasticity of their meaning and function, which radically altered in nature. The Moroccan manuscripts were transformed from precious material objects to be exchanged, into repositories of knowledge to be studied, and into doctrinally problematic texts to be stored or burned, and then again into political hostages in return for which the Moroccan Sultan was willing to pay. The second part of paper examines the circulation of the Moroccan manuscripts in relation to what censors, inquisitors and scholars in seventeenth-century Spain thought about Arabic books and knowledge. Finally, the third part examines the legal debates that followed the library’s capture, when the collection became the locus of trans-Mediterranean negotiations over sovereignty, Maritime law, captives, and banned knowledge. By placing and analyzing the journey of Zidan’s books within the context of Mediterranean history, the paper explains (1) why Spain established one of the largest collections of Arabic manuscripts exactly when it was cleansing its territories of Muslims, and (2) why the Moroccan collection was kept behind locked doors at the Escorial.