I just learned over on the Nikon Rumors website that there is an app for downloading and viewing your Nikon camera manual on your iPad or iPhone. The app, called Nikon Manual Viewer, can be downloaded for free. Once you have downloaded the manual, you can then view it offline at anytime. It does not appear that Canon has a similar app yet.
Be sure to also consider my dSLR camera guides, which can also be downloaded and carried with you on your iPad or computer, including Nikon D7000 Experience and Nikon D5100 Experience. These guides go beyond the manuals to help you learn not only how, but more importantly when and why to use the features and functions of these versatile cameras.