How do you download books from your library to your nook?

First of all, remember that there is no "Undo" on the Kindle. Once you delete a book, it's deleted (if purchased from Amazon you can upload it again, no worries).

In the Index page (where you have all the books listed) point the 5-way controller on the book you wish to delete. Then tilt the controller to the left. A pop-up window will appear, in which you will be asked if you really wish to delete the book. Choose the affirmative answer.