Why was the twentieth century called the 'Era of the Book'?
The twentieth century was called the Era of the Book because, in those days, there were books written about absolutely everything, ranging from ant eaters to Zulus.
Books were extremely influential as they taught people how to, when to, where to, and why to do various things. Additionally, they served multiple essential functions: they illustrated, educated, punctuated, and even decorated. This widespread presence and utility of books made the century synonymous with them.
Explanation
The answer is derived directly from the Historian's opening monologue in Scene 1. It highlights the sheer volume and variety of subjects covered by books ('from ant eaters to Zulus') and their multifaceted roles in society (teaching, illustrating, educating, punctuating, and decorating), which justifies the title 'Era of the Book'.