Here I share the nice analysis done by HGCenty showing that Plarium has been increasing champion book requirements steadily over time.
He used data mined information to get as accurate picture as possible and, on top of that, he separated Legendary and Epic champions.
He plotted the data on a chart showing the # of books required to max a champion versus the champion ID which is a proxy for when the champion was released and this resulted in the chart showing that champions released later on require more books.
Chart of Legendary champions
Chart of Epic champions
