"It doesn't matter what you collect as long as you do it passionately," Wilmarth Sheldon Lewis once said. "Passion," however, is perhaps too mild a term to describe Lewis's drive to collect. Few if any collectors have been as obsessed with their subjects or have pursued them as ardently as Lewis in his quest for everything related to the eighteenth-century man of letters Horace Walpole. A key figure in the latter half of the 1700s, Walpole fostered a Gothic revival in architecture with the remodeling of his Twickenham villa, Strawberry Hill. He wrote the first Gothic novel, The Castle of Otranto (1764), operated his own printing press at his estate, and wrote thousands of letters revealing of contemporary English culture. As Leslie Stephen observed, "The history of England throughout the eighteenth century is simply a synonym for the works of Horace Walpole." Having committed himself to pursuing Walpole, Lewis...