huh, I'd bet there are authors that would give a body part to have a book at that ranking on Amazon.The 3E PHB was published August 10, 2000, while the 4E PHB was published December 5, 2008. That's 8 years and 4 months.
Hoard of the Dragon Queen was published August 19, 2014.
That means it will hit the combined publication lifetime of all of 3E at Christmas this year. Currently it's only the #6,096 selling book on Amazon...but it is still selling, and you can find it in stores. That's pretty shocking, all things considered.