Books sales are sales of anything with an ISBN. This includes adult colouring books, which I’ve heard is responsible for possibly all the sales increases in the past few years.
After a flurry of activity in the last decade, I haven't heard anyone mention adult coloring books even once in the past two or three years. As long as we're all baselessly speculating, I will baselessly speculate that they were just an ordinary fad. My own anecdote is that disillusionment with internet content and electronic consumption in general has caused me to buy and read more books recently than at any point in my life.