Final answer:
The rule definition in question describes a cross-selling marketing strategy, not tying sales. Tying sales involve forcing a customer to purchase an unrelated product in order to obtain the one they initially wanted, which is not the scenario described.
Step-by-step explanation:
The rule definition provided does not match the concept of tying sales. Tying sales occur in a scenario where a customer is compelled to buy one product only if they also purchase another, unrelated or only loosely related product.
In the case of the rule definition, which suggests showing customers related books after they've placed a book in their shopping cart, it is more akin to a marketing strategy known as cross-selling. Cross-selling aims to increase the value of a customer's purchase by suggesting additional products that complement their initial choice, rather than requiring the additional purchase as part of the sale.