When Bitcoin is making a bull run, there will always be good coverage in the mainstream media and that can mean more exposure for the coin, hence there will be a spike in people's interest with the subject resulting them to look for books that can be found in the Amazon Marketplace. This is just a natural reaction. However, when BTC is tumbling down and is getting trampled on there can be a loss of interest...this is just like a see-saw type of thing.