All rumors about delays are gone with the wind. This morning I got an email from a Blizzplanet visitor, confirming Diablo III: Book of Cain is also on sale at local bookstores in Singapore.
Seems there were delays after all, at least on Amazon's end. As Bumblebeesoup stated in the comments, Insight Editions acknowledged via Twitter to know about the delay. This has nothing to do with the January 10th release date posted by Amazon. As of Sunday, Dec 11th, the Publicity Manager told Blizzplanet that Book of Cain was on track for release on December 13th. This delay happened overnight and didn't exist prior to this past Sunday.
"We expect the pre-orders to ship by Friday (16th) of this week and in most stores by the weekend." -- source
"It should definitely ship out before the 19th of December!" -- source
"The original pub date was 12/13. It's been delayed a few days. Our apologies." -- source
"We expected it by the release date as well. We appreciate your patience. Pre-orders help determine how many to print." -- source