The Holy Hierarchy is the word of fifty angels, that has been translated into a human language.
It can be read at: https://holyhierarchy.neocities.org/read
The Hypernia is the detailed account of the revelations themselves and the stories of how they were revealed. The Hypernia is written by a human, and not the angels. The Holy Hierarchy is the only book which is necessary for the Hierarchists, the Hypernia is simply optional and written by the prophet of Hierarchism. The Hypernia can be read here.