Opportunity Cost and the Mystery of the Left Socks Lost is the third book in a series to aid in teaching economic concepts to children from 1st-4th grades. The series is about several neighborhood kids who wish to buy new toys and want to mimic the hero Gabriel David from the first book, Supply and Demand and the Lemonade Stand. In this book, left socks have been disappearing so Abigail Alexis embarks on becoming a private detective to solve the mystery and earn her new toy. The book is written at an accessible level, and it is encouraged to be used in grade school classrooms where economics is a part of the curriculum. 

There are two companion books as well – Supply and Demand and the Lemonade Stand, and Competitive Little Lads and Ladies.