Publishers have gone to great lengths to protect the contents — especially the ending — of the last Harry Potter book until its official release on July 21. Now, so too is the world’s largest retailer.

Starting this week, Wal-Mart Stores said a million Wal-Mart employees are making a pledge that once “Harry Potter and the Deathly Hallows” is released at 12:01 a.m. July 21, the final chapter of the book will not be discussed at cash registers.

For fans still worried about prematurely hearing the ending, Wal-Mart said some stores in cities “where a particular Passion for Potter seems to exist,” it will hand out ear plugs for those who need to find a “quiet corner” to read the book.

But Wal-Mart doesn’t want to be the only one keeping its lips sealed. It is asking Potter fans to take the Harry Potter Pledge on line at http://www.makethepledge.net and agree to keep the secret. And just in case those taking the pledge have not already lined up an order for the popular product, once they take the pledge, Wal-Mart then conveniently provides a link to preorder a copy of the book from the Wal-Mart Web site.