The dispersal of the library amassed by George Spencer-Churchill (1766–1840), Marquess of Blandford and later fifth Duke of Marlborough, is most commonly cited today as a preservative against folly. The collection contained some of the most sought-after incunabula of a period defined by the high prices paid for early printed books. It included a fine selection of Caxtons, spectacular botanical and emblem books, and the iconic Valdarfer Boccaccio - the first edition of the Decameron, purchased by Blandford in 1812 for the unprecedented sum of £2,260. The Boccaccio was symptomatic of the profligate expenditure of its buyer. By 1819 his spendthrift ways had ruined him, leading to the sale of his opulent estate at Whiteknights, near Reading, and the dispersal of one of the key libraries in the era of bibliomania. Reissued here together are the two parts of the auction catalogue, both annotated by an auction attendee who recorded details of the purchasers and the prices paid. Ed Potten, Head of Rare Books at Cambridge University Library, has provided a new introduction that places the catalogue in its wider context.