How a Europe torn apart by the Napoleonic Wars became the site of a book-collecting frenzy, laying the foundation for the Anglophone world's greatest collections of rare books. At the beginning of the nineteenth century, bibliomania ('book madness') swept the English upper classes. Vast sums of money were spent on books from the dawn of printing; fortunes were lost, families shattered, estates sold. By the end of the century most of the collections assembled had been sold or donated and had come to rest in national and research libraries across the UK and America, forming the groundwork on which almost all collections of early printed books in the Anglophone world have since been built. But where did these books come from? Who bought them and how? Portrait of an Obsession recovers this forgotten story by focusing on the obsessive and majestic collecting of George John Spencer, 2nd Earl Spencer (1758-1834), the greatest of the Regency bibliomanes. Historian, librarian, and modern-day bibliophile Kelsey Jackson Williams explores how Spencer combed a Europe torn apart by the Napoleonic Wars for rare books, using agents and runners (including a British spy) to intimidate librarians, ransack monastic collections, and deal in stolen and looted goods to form his great library at Althorp. The catastrophes of war allowed for unparalleled opportunities, if one had the resources and drive to capitalise on them. Through Spencer's life and collection, which is now part of the John Rylands Research Institute and Library in Manchester and contains some of the rarest books in the world, Portrait of an Obsession unpicks the extraordinary psychology of Georgian book collecting. In so doing, it asks how Georgian collectors came to own such eye-watering arrays of rare items--and what that means for the Anglophone museums and libraries in which they sit today.