Surprisingly early, as it happens. The earliest examples of bookbinding are from India before the Common Era, where a sutra (from the word for "a rope or thread that holds things together") was made from palm leaves, written on, then sewn together along one edge; enclosed in wooden boards, the excess thread was wound around the boards to keep all together. Buddhist monks carried this technique through Persia, Afghanistan, Iran, until it reached China in the 1st century BCE.
This method spread slowly, but by the 4th century CE bound books and scrolls were being produced in equal numbers (based on surviving texts). Using wooden boards to contain information was not a new idea. Wax tablets in a wooden frame existed from Roman times right through the Middle Ages (as mentioned here). The word "book," however, was not being used the way we use it today. Isidore of Seville (c.560-636), the famous early etymologist, said:
A codex is composed of many books; a book is of one scroll. It is called codex by way of metaphor from the trunks (codex) of trees or vines, as if it were a wooden stock, because it contains in itself a multitude of books, as it were of branches.As for the word "book" itself, which eventually replaced the word "codex," its likeliest origin is in the word for beech. This has led scholars to assume that some of the earliest collections of writing (that weren't on scrolls) were written on beech wood.
The important point is that the development of the codex method of holding data increased the longevity of "books" over scrolls, which did not have hard covers and which experienced wear every time they were unrolled and re-rolled. "Books" could now be made larger, as well, without falling apart; meaning, for example, an entire Bible could be contained in one printed object.
Were there differing opinions on the usefulness or appropriateness of the different styles of "book"? Perhaps there were, although any difficulties in adapting to the new style probably did not go like this.