Most Nikon manuals are downloadable from your local Nikon support website in PDF format. I'm sure it must be possible to get a free PDF reader app for an Android device - your phone may have one already
Since the Nikon manuals are in the PDF format, you might want to get the EZ-Pdf Reader. It works on Droid devices including the Nook Tablet and Nook Color eReader. This app supports bookmarks and commenting.
There are even ePub readers available for the Droid and the Nikonians Press books can be purchased through O'Riley Media in the PDF and ePub format, and for a few dollars more you can include the paper copy. O'Riley media also provides updates for registered books.