For me it was The Black Swan by Nassim Nicholas Taleb.

A "Black Swan" is an improbable event that has a major impact, and happens when people least expect it (e.g.: 9/11, WWI, 2008 economy crisis).

Black swans can have a negative or positive impact. The negative ones can wipe out things build over decades in a matter of seconds, while the positive ones can take longer (e.g.: the advancement of technology in the past 30 years). The positive ones need to be put on scale with longer intervals of time. They’re harder to notice if you look at a specific time in history.