I was interested in this book after previously reading by Lewis "The Blind Side" (yep, that one), "Moneyball", "The New New Thing" and "Home Game: An Accidental Guide to Fatherhood". Good books all with the most recent review linked to here.
"The Big Short" is ostensibly about a handful of people that got rich betting against the financial hubris that preceded the real estate crash at the end of the '90s, but it's also about the mechanisms that made them wealthy.
Lewis writes about a highly complex financial system that's designed to be... highly complex and "above the common man", but does I'd say as good a job as could be done explain all the instruments of the financial industry around subprime mortgages.
Mortgage backed securities, Collateralized debt obligations (CDOs), credit default swaps... all of them tools that were created to either continue riding the wave of rising home values or to bet against that prospect. Where the individuals Lewis features made their money was by basically betting that consumer home loans would have higher incidences of default than predicted by those selling the CDOs and credit default swaps.
The way it all worked as described by Lewis was pretty fascinating in that you had rating agencies like Moody's and Standard and Poor's responsible for setting the values of the credit default swaps on pretty much arbitrary or made up terms. Even worse than the ratings agencies were the financial institutions packaging the loan bunches and continually manipulating the contents to make the loan pools appear less likely to have defaults than they actually were.
The whole thing is described by Lewis as being a crazy system designed by people who figured it would go up forever. However, it didn't and the few contrarians who (to borrow a phrase) "saw the emperor had no clothes" got very wealthy very fast.
All in all, the Lewis book is entertaining and as easy reading as possibly could be given the high finance topic.