It's possible this book may be included in the Collector's Edition, however with the release date of the book set at November 15th, I almost don't see this happening. The reasoning behind this is simply because anyone who purchases the CE will not really be getting what they've paid for: Collectibles. This particular item would not really be considered a collectible, as it can just be attained by anyone, separate from the CE itself, prior to the game being released. Usually Blizzard tries to keep items in the CE unique, so those of us that prefer to purchase the CE, can feel like we're actually obtaining some awesome goodies.
Official Blizzard Quote:
In Blizzard Entertainment’s Diablo® and Diablo II, the recurring character of Deckard Cain delivered quests, accompanied the brave adventurer, and, as the last of the Horadrim, provided a link to the greater history of the world of Sanctuary. Ever mysterious during these appearances, Cain hinted at a larger story, providing snippets of it in his notebook. Diablo III: Book of Cain is Cain’s formal record of this greater tale—a dissertation on the lore of the Diablo universe, told by one who has witnessed and participated in some of the epic events that make up the eternal conflict between the High Heavens and the Burning Hells.
Designed as an “in-world” artifact from the Diablo universe, Diablo III: Book of Cain includes Cain’s revealing meditations, as well as dozens of sketches and color artworks depicting the angelic and demonic beings who wage constant war with one another.
For players of the eagerly anticipated Diablo III, this mysterious tome will provide a deeper understanding of the franchise and its storyline.
Thanks to Guarav for the tip!