The books contains knowledge on both the Darkhallow and how to wield the power of necromancy over Black Court vampires. If the Heirs of Kemmler got their hands on it, the knowledge in it could create four or five "Kemmlers", creating havoc.
Harry Dresden found it with Waldo Butters' assistance at the Field Museum of Natural History under Sue's skull. When Grevane arrives, Harry flips through it so that Lasciel can memorize it. Grevane took it from Harry after he overcame him. Harry uses the information in it to re-animate Sue in order to stop the Heirs of Kemmler.
Evil Bob: The Word of Kemmler contains: "Knowledge. Truth. Power. The master wrote down his teachings, wizard, so that those who came after him could learn from him. Could learn about the true power of Magic. What you call magic is nothing more than parlor tricks beside the power to master life and death itself."