I think it was in the mid-to-late 90s as the internet was just becoming more available to lower income groups.
"Nobody buys physical encyclopedias anymore, everything else is going to go that way soon too"
In high school debated on the side of the demise of books (2002) and I won. Back then I convinced myself of my side for the sake of my debate performance, but I'm pretty sure today that books are going nowhere in a hurry.