Book sellers are champing at the bit for the July 21 release of “Harry Potter and the Deathly Hallows,” the seventh and final installment of J.K. Rowling’s wildly successful fantasy series, which has sold 325 million copies worldwide.

And with some industry experts expecting Harry Potter 7 to become the fastest-selling title of all-time, an honor currently held by the sixth installment, “Harry Potter and the Order of the Phoenix,” retailers are looking to capitalize by offering a slew of early discounts for customers pre-ordering the book.

The pre-orders ensure hard-core fans will be able to get the book the day it comes out, helping them to avoid the inevitable disappointment that will ensue in the event book 7 is sold out, said Al Greco, a professor of marketing at Fordham University and an analyst for the Book Industry Study, which produces an annual study of book sales.

The pre-orders also function as a key harbinger of demand, enabling publishers to know how many books they will need to print.

“When Harry Potter 5 came out, they did not have enough printed copies to meet demand and Scholastic had to go back to press and print more copies,” Greco said, adding that it will be heavily discounted.

Amazon.com Inc.’s early returns suggest demand will be higher than ever. 

The online retailer said Friday that first-day advance orders rose 547 percent from book six and is offering early buyers the book for $18.89, 46 percent off the list price of $34.99 

Wal-Mart Stores Inc. is offering pre-orders for $17.27

Meanwhile, Barnes & Noble, the top U.S. bookseller, is offering Harry Potter 7 for $20.99, or 40 percent off, but is allowing members of its awards program to order it early for $18.89.

Borders Group Inc., the second-largest U.S. bookseller, is selling pre-orders for $20.99, while wholesaler Costco Wholesale Corp. is also selling it for $18.99.