Ma'at is the Egyptian goddess of justice. Her symbol is the ostrich feather you can see in the cover.
According to Egyptian mythology, when a person died, s/he was judged in a very simple way: Anubis would put his/her heart (ib) on top of a scale, and then place the Ma'at feather on the other side. If the soul of the person was pure, the scale wouldn't move one way or the other. The person would then go to some sort of paradise, which was nothing but Egypt minus the hardships - hard work, diseases, etc. But if the heart was sinful, it would weigh more than the feather, and a demon called Ammit would eat up the person's heart, condemning his/her soul to be restless forever (or destroyed, according to an alternative version).