In present-day U.S. publishing, people often use the term content to refer collectively to the editorial and design elements in a periodical or book. And yet, as DJ Far observes, the front-matter listing of the book or periodical's editorial content is usually called the "Table of Contents."
The Ngram chart for "table of contents" (red line) versus "table of content" (blue line) for the years 1650 through 2000 shows that there really isn't much of a contest here:
And yet a match of "contents of the book" (red line) and "content of the book" (blue line) for the years from 1700 through 2000 shows a much closer split in usage over the past century:
It's an interesting phenomenon, but I have no explanation for why it has emerged.