What is the theme in The Big Short: Inside the Doomsday Machine by Michael Lewis?

Investing is the act of buying stock in a company and gaining a profit should the price of the stock go up before you sell it or losing money should the price go down. The same is true in bonds, but the difference is that the investor does not always have a clear idea of how much a bond is worth because unlike stock prices, the price of a bond is not shown on a ticker tape display and updated every few seconds.