What do you think is the most popular book sold right now? Harper Lee’s hugely anticipated second novel, Go Set a Watchman? George RR Martin’s Song of Ice and Fire fantasy series? No and no. It’s a coloring book.. No, not for children… it’s for adults. We had no idea too that adults still play with this but apparently they do. Maybe it;s some kind of stress relief you know? It can be stressful to be an adult with all the kids running around and screaming.