In this Usul discussion, the Book of Allah means the Noble Quran:
• Allah sent it to Prophet Muhammad ﷺ through Jibril عليه السلام.
• It reached the Ummah through the Messenger of Allah ﷺ.
• Its Quranic wording reached us without disagreement through continuous Tawatur.
This definition identifies both the revealed text and the way its wording reached us.