Roughly a third of all the paragraphs Grisham has ever written deal directly with the legal system, and similarly Steel has given almost an exact mathematical third of her pages over the years to the theme of domestic life, or even more specifically ‘time spent inside the home.’ There are several significant things to say here. The authors are known for their signature topic, and fans expect them to deliver it. If that trademark gets a third of the book, the book-per-year writer then has two-thirds of his or her space to introduce the tangential topics that make each new book seem a little different. This formula can go on forever, and we will see how cleverly this is done: for now the point is in the proportion— one-third the same, two-thirds different.
We notice this rule at work on Instagram as well; can only break the mould with a slight variation.