Zamorak cast a spell on the book while they were watching Char's performance when she was a dancer, which altered the spell Zaros had placed on the book of Perjour, duplicating the book into Zamorak's possession, so Zamorak could read what was being reported back to Zaros.
When Perjour was taken with Zamorak, Viggora, Thammaron, Enakhra and Char on a mission, Zamorak altered the spell again, causing the book to show a different version of events, hiding many occurrences. This alteration was made to help him overthrow Zaros with the Staff of Armadyl, as Zaros would not know of Zamorak's actions, enabling him to plan unhindered.
Perjour died in the caves leading to the Elder Kiln, as it shown in the Journal of Perjour, obtained during The Elder Kiln quest. During his last writings, Perjour stated that he was searching for the Kiln in attempt to raise an army for Zamorak, so it is conclusive that he sided with Zamorak during his rebellion and served him until his death.