Reach the part of the game where Niko and Roman have a safehouse in South Bohan. 
In the house on the left of the table near the television is a pile of books. The
middle purple book of a stack of three is titled "Madness & Blasphemy: The Do's 
and Dont's". This is an reference to the movie 300 where in the "This is Sparta!"
scene, the lines "This is blasphemy! This is madness!" are spoken.