"The shelf life of the average trade book is somewhere between milk and yogurt"

This quote by Calvin Trillin is a funny method of saying that the life-span of a trade book is relatively short. He is comparing the shelf life of a trade book to that of milk and yogurt, two products that have a relatively short service life. Milk usually lasts for a few days, while yogurt can last for a few weeks. In contrast, the typical trade book has a service life of a few months or perhaps a couple of years. This is because books can end up being out-of-date rapidly, as brand-new info is constantly being released. Additionally, books can become less popular in time, causing fewer sales and a shorter service life. Ultimately, this quote is an amusing way of stating that the life span of a trade book is reasonably brief.