A Book Index Is a Work of Art, a Nest of Clues to the Essence of the Work

From a review on theamericanscholar.org by Charles Trueheart of the book by Dennis Duncan titled “Index, a History of the: A Bookish Adventure from Medieval Manuscripts to the Digital Age”:

To understand what an index is, it may help to begin with what it is not.

An index is not a table of contents—such a table would feed multitudes—but it does lay out in more granular form all of the contents of a book. An index is not a concordance, strictly speaking, although they are cousins. A concordance reverentially locates every instance of every significant word of a text—the Bible, originally.