The Library contains some 3,500 books, many of which survived the fire of 1871, and others that were subsequently brought from Chatsworth. Among them are works by the distinguished scientist Henry Cavendish (1731-1810), grandson of the 2nd Duke of Devonshire. His microscope can also be seen in this room. Henry Cavendish discovered nitric acid, the properties of hydrogen, calculated the density of the earth and undertook many other important scientific experiments.
With renowned Cavendish taciturnity Henry denied the pleasures of social life and lived the life of a recluse. In his house overlooking Clapham Common he had a special staircase added in order to avoid seeing his servants; if he caught sight of a serving maid she was instantly dismissed. His mysogynism was legendary, and he never married. Due to his indifference towards money and his frugal habits, his assets accrued un-disturbed, so when he died he left the great part of an exceedingly large fortune to Lord George Augustus Cavendish, who had been allowed by the scientist to visit him for half an hour every year.
The craftsmanship of the linenfold panelling, of which there are four variations at Holker, and the plaster moulding of the ceiling are of excellent quality - as they are throughout the house. Electricity was installed in 1911 and care was taken to conceal lighting. Switches were also hidden; in this room they can be found behind imitation books, each bearing a humorous title, at the side of the doorway.
Amongst the photographs of the family in the Library is one of Harold Macmillan, Lord Stockton, and his wife Lady Dorothy who was born at Holker, daughter of Victor, 9th Duke of Devonshire.