The Rocky Nook Nikon books are also available on the Barnes & Noble Nook readers, both the B&W Nook and the Nook Color.
There is an option to also obtain the PDF version of the ePub.
The Nook Color has been updated to include the ability to run Apps. And there is ez PDF Reader that will put all of your PDFs into an application contained library. So you can not only have the Nikonian books, but also the Nikon manuals available in the field.