Morning Star (Red Rising #3) by Pierce Brown

Darrow is a Helldiver, one of a thousand men and women who live in the vast caves beneath the surface of Mars, generations of people who spend their lives toiling to mine the precious elements that will allow the planet to be terraformed. Just knowing that, one day, people will be able to walk the surface of the planet is enough to justify their sacrifice. The Earth is dying, and Darrow and his people are the only hope humanity has left.

Until the day Darrow learns that it is all a lie. That Mars has been habitable - and inhabited - for generations, by a class of people calling themselves the Golds. A class of people who look down at Darrow and his fellows as slave labour, to be exploited and worked to death without a second thought.

Until the day Darrow, with the help of a mysterious group of rebels, disguises himself as a Gold and infiltrates their society.