The 2-volume "Syntopicon" of Britannica's Great Books of the Western World, providing a survey and a concordance of the Great Ideas treated in the remaining 51 volumes of the set, is itself one of the most important works of the 20th century.

One of the criticisms I've read of the Great Books series is that it is little more than a "reading list" of important works of literature. That criticism might have some justification, if it were not for the Syntopicon, which orients the reader to the content of the Great Books, breaking down the entire set into its 102 component "Great Ideas"—the key topics addressed by Western literature as a whole since its inception.