See update to this story (10/2/2013)

Ebooks fans may have noticed that some titles are no longer available for wireless downloading via Amazon.  This is due to restrictions recently put in place by Penguin Book Group.
 
What does this mean for you? Kindle format ebooks published by Penguin Group can no longer be delivered using wi-fi to a Kindle device or to a Kindle app. The titles can only be transferred to a Kindle device using a USB connection. Amazon states that "these restricted titles also may not be accessed on Kindle reading applications." This means if you have a Kindle app on another device, such as an Android tablet or an iPad, you will not be able to access the restricted Penguin titles using Amazon.
 
If you use an Android tablet, an iPad, or other mobile device, you can still access our ebooks by downloading the free Overdrive Media Console app and checking out ebooks in the more flexible EPUB format rather than the Kindle format. For help installing the Overdrive Media Console app on your mobile Android or iOS device, please see these helpful videos.