Piketty's book (Capital in the Twenty-First Century) is most definitely not a fad. It is a result of serious research into historical economic data and anyone intellectually honest ought take it seriously. As anything in the social science of economics, it is subject to debate, but to call it a 'fad' is odd, to put it mildly.
I think it's popularity among layfolk, like myself, was a fad. I'd be incredibly surprised if it's flying off the shelves now like it was right after release.
But popularity doesn't validate or invalidate its content.
