Try this first, pres and hold the power button on yoir nook for 20 seconds, thrn power back on and try reading the book. If that fails tty this next....To get the book out of the Kindle App, tap the “Actions” button once more. This time, a menu will appear and will say, “Delete from device.” Tap this. Open your Kindle App and tap the sync button (lower right hand corner). The book should disappear from within your Kindle App. If it does not, you can hold your finger for a few seconds on the book. A menu should appear that says “Remove from device.” Tap this, and the book should disappear.
Then try downloading again.
Go into your Amazon account at Amazon.com and tap on “Manage my Kindle.” You will see a list of things within your Kindle App. Your Library book will be listed there” To the right, you will see an “Actions” button. By tapping the button, a menu will appear. On this menu you will see delete, Tap it. Then redownload.
Then on the kindle app, If you do not see your book within the Kindle App, look to the upper right hand corner of the App. You should see a half circle with an arrow on it in shaded grey. This is the sync button. Tap it, and your book should download. Open your book by holding your finger for a few seconds on the title.