Why finance? Finance was the major viable option for people with your background but that's not the case anymore. Every industry needs people with your background -- more than ever!
However, if you're hell bent on going into finance and you're going to read one economics book, read Thomas Piketty, "Capital for 21st Century".
These are the classic economics tomes that you'd read over a lifetime:
Karl Marx, "Capital"
Adam Smith, "The Theory of Moral Sentiments"
Adam Smith, "The Wealth of Nations"
FA Hayek, "The Road to Serfdom"
However, if you're hell bent on going into finance and you're going to read one economics book, read Thomas Piketty, "Capital for 21st Century".
These are the classic economics tomes that you'd read over a lifetime:
Karl Marx, "Capital" Adam Smith, "The Theory of Moral Sentiments" Adam Smith, "The Wealth of Nations" FA Hayek, "The Road to Serfdom"