Tokyo Ghoul: Past

Tokyo Ghoul: Past is the third novel set in the Tokyo Ghoul universe. The third novel tells stories from before the events of the manga Tokyo Ghoul.
Before the steel frame incident in the 20th Ward, there were reports of Ghouls lurking among the masses in Tokyo, carefully stalking their prey. This third novel covers the events that occurred before the opening act of Tokyo Ghoul—a time when Kaneki was still human, the Kirishimas still lived together, and Rize enjoyed her feasts.
(Source: Tokyo Ghoul Wikia)