Mention of gold and silver investment in books

This graph shows the frequency of appearance of the phrases "gold investment" and "silver investment" in 5 million books over the past 6 decades. At present moment, it looks like we're still quite far away from the bubble period of 1979/1980. Lots of books about these investments have been published right after the bubble burst in 1980, as we can see from the graph. Maybe lots of authors were late to the party, considering that it takes a few months to publish books.