Comet Home Page
Whipple, F. L. 'The Mystery of Comets"
Whipple p76 - without some mechanism to replenish them, "eventually all will be lost."
Whipple p83 - examples of comets disappearing
Sagan, C. and Druyan, A., "Comets"
Sagan p194 - comets disappearing
Sagan p201 - Oort Cloud not proven