Where does the "1/18th of the book" come from? The medium article has a "(18/52)" in the title, but that rather seems to indicate that the article is number 18

Where does the "1/18th of the book" come from? The medium article has a "(18/52)" in the title, but that rather seems to indicate that the article is number 18 in a series of 52 (weekly book reviews in a year or so).