I walked into a Barnes & Noble (NYSE:BKS) earlier this week, only to nearly bump into a huge display with dozens of copies of Harry Potter and the Deathly Hallows. My initial reaction -- well, after reminding myself that I need to watch where I'm going -- is probably the same question that a lot of superstore customers have been asking themselves lately: Who's going to buy all of these books?

We're now four weeks removed from the Midnight Magic costume parties held at Barnes & Noble stores all around the country. We know that the book blazed out of stores early on. Amazon.com (NASDAQ:AMZN) moved 1.4 million copies the day the book hit the market. Borders (NYSE:BGP) set a one-day record, too, moving 1.2 million copies of the seventh and final book in the celebrated J.K. Rowling fantasy series. Barnes & Noble issued a press release the following day, promoting its having sold 1.8 million copies of the book in 48 hours.

Scholastic (NASDAQ:SCHL) -- the company blessed to be the stateside distributor of the series -- moved 8.3 million copies the day the book hit the market. The media moved on after the Potter buzz subsided, but Scholastic's press releases kept coming.

It wound up selling 11.5 million copies over the first 10 days. The frenetic pace found it printing another 2 million copies to follow its initial run of 12 million. It had to. Retailers were running out. However, if your Barnes & Noble is anything like mine, you know that they have plenty in stock now.

Naturally, this may lead one to wonder whether Scholastic will get hit with retailer returns. It may not seem that way just yet. Even though the vegan-vampire-powered Eclipse went on to temporarily topple Potter from the bestseller list's pole position, Potter isn't going away. This morning, Potter is still the top-selling title on Amazon.com, and second only to Eclipse on Barnes & Noble's website.

Hermione stranger
One can also argue that Scholastic is being conservative in its release. There are just 14 million copies of the book out there, less than half of the 29 million copies that Scholastic has printed over the years of the book that started it all, Harry Potter and the Sorcerer's Stone.

Harry Potter book U.S. books in print 
Sorcerer's Stone  29 million
Chamber of Secrets  24 million
Prisoner of Azkaban  20 million
Goblet of Fire  19 million
Order of the Phoenix  17 million
Half-Blood Prince  17 million

Deathly Hallows

 14 million

Potter fans may notice an immediate trend: the older the book, the wider the circulation. This may come as a surprise to those who follow only the release-day headlines, since booksellers always seem to be bragging about pushing a record number of Potter books with every subsequent release. When Barnes & Noble hit the 1.8 million mark in the final book's first two days on the market, it was a 38% improvement over the number of copies it sold of Rowling's previous book in that time.

That is important, because a cynic eyeing the cumulative numbers would be wrong to assume that the series is waning in popularity, just because there are fewer copies out there of every subsequent book.

So everything is peachy, right? Well, not exactly.

Wrong Weasley
Why am I worried about the potential wave of returns down the line? Well, part of the allure of a book series is that each installment renews interest in earlier titles. The mere release of the seventh book was enough to get the sixth book back on the bestseller list. There won't be an eighth book.

Don't get me wrong. New generations will come around and enjoy the classic series. Time Warner (NYSE:TWX) also has another two films to put out in the series, kicking up sales of the final two books. Universal Orlando's Islands of Adventure -- an amazing theme park owned by financial fat cats General Electric (NYSE:GE) and Blackstone (NYSE:BX) -- is building an ambitious Harry Potter-themed island expansion within the park. So Potter isn't exactly going away.     

My point is only that booksellers need to be realistic. There will no longer be a call for the store-level celebrations. Another franchise will come around to capture the imagination -- and pocketbooks -- of young readers. It's inevitable.

The only real challenge for whatever "the next big thing" to come down the pike will be seeing whether it can even approach the magic of the Potter franchise. It probably can't. Goldman Sachs upgraded shares of Borders this morning, though the industry's got more questions than answers at present. Perhaps that is why the upgrade simply bumps up Goldman's "sell" rating on the stock to a slightly less lukewarm "neutral."   

It's certainly nothing to get excited about. The wave of emotional apathy will eventually carry over to the Potter franchise. It's nothing against Rowling's stellar achievement. When you move 350 million books globally, you'll always command respect. However, it's undeniable that Potter will be a little less relevant, now that the spigot of freshness has been turned off.

Demand for the series will taper off without the nearly annual catalysts that sprung the series to leafy life. In short, the next time I bump into a Harry Potter display, it just won't be all that massive.

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Longtime Fool contributor Rick Munarriz didn't read all of the books, though he did go through the last few chapters of the final book so he could engage in literary discussions with his wife and son. He is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. He does not own shares in any of the companies in this story. The Fool's disclosure policy has a Patronus shaped like Warren Buffett.