Nothing unique. If Amazon had existed in the 70s and someone had gathered the same data, I'm sure you'd see the same shape. In fact, the publishers and bookse

Nothing unique. If Amazon had existed in the 70s and someone had gathered the same data, I'm sure you'd see the same shape. In fact, the publishers and booksellers probably did gather that data, so it ought to be possible to check whether this is true or not.