The central thesis of Piketty's Capital[1] is that inequality is a core feature of capitalism. He backs this with meticulous research[2] into centuries of wealth, income, and tax data. You can argue about what is the "right" level of inequality or with the policy recommendations (that massive government interventionism via a global tax on wealth is required). However, the results and data are solid.

[1]: https://www.amazon.com/Capital-Twenty-First-Century-Thomas-P...

[2]: http://piketty.pse.ens.fr/files/capital21c/en/Piketty2014Fig...