
A random walk down Wallstreet
Options and volatility pricing, by Natenberg
Why stock markets crash, by Didier sornette
Manias, panics and Crashes by Aliber and Kindleberger
Debt: the first 5000 years
tl;dr - Stock markets crash because of fractals/chaos/complex systems. And there's no such thing as an equilibrium (in the stock market) because if there was, everyone would agree on the true value of a stock and no one would buy or sell for that reason.
Why Stock Markets Crash by Didier Sornette The complete oeuvre of Paul Wilmott
(The Computational Beauty of Nature, by Gary W. Flake because it's wonderful and puts you in the right frame of mind)