Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 178.
When thinking about the relationship of the New Testament writings to one another, we must constantly bear in mind that in the ancient world books were not published as they are today. In the modern world, books are mass-produced and sold all over the world, with the distribution of copies taking weeks at the most. In the ancient world, books were copied one at a time, and distribution was haphazard at best. In-house literature was not advertised, and circulation was random and uncontrolled.