Blizzplanet reports the Diablo III: Book of Tyrael pre-orders have begun. Insight Editions and Blizzard Entertainment plan to launch this book on October 1, 2013.
The book is rumored to herald the announcement of Diablo III expansion at this year's BlizzCon. The convention will take place at the Anaheim Convention Center on November 8-9th.
Part of the book description hints at an expansion:
"Here, in this illustrated and comprehensive tome, Tyrael reveals never-before-known secrets about the history of the world, the [dark threats that yet face mankind], and his decision to join the ranks of humanity as a mortal."