I have the full original set of Great Books. I know there was an updated version published in the 70s(?). Is there a way to attain the rest without re-buying the whole set?
The updated set was published in 1990. Wikipedia details the exact differences between the two editions. To summarize: the primary changes are the addition of six volumes of 20th century material - volumes 55 through 60. There are also smaller changes (addition or deletion of one or two works - the works deleted are Apollonious' On Conic Sections, Sterne's Tristram Shandy, Fielding's Tom Jones, and Fourier's Analytical Theory of Heat) in volumes 20, 23, 31, 34, 43, 44, 45, 46, 47, 48, and 52. For more exact details, again, see the Wikipedia article. It seems to perfectly have the new set, you'd have to buy 17 volumes.