When author J.K. Rowling said that the last Harry Potter book, Harry Potter and the Deathly Hallows, was the finale for the series few people believed her and many fans expected she would find a way to revisit the fantasy world she created.

Rowling has mostly avoided writing about the boy wizard, aside from some stories under a pseudonym on her Pottermore website, but she did not leave the wizarding world entirely behind as she has agreed to work with Time Warner's (NYSE:TWX.DL) Warner Bros. on three movies based on her Potter spin-off Fantastic Beasts and Where to Find Them (Scholastic: (NASDAQ:SCHL)). Unlike the Potter series, which yielded eight movies from seven thick tomes, Beasts comes in a slim 128 pages.

The book is not a novel but a sort of textbook that's referred to in the Potter series as being something that every wizard's home would have -- a kind of reference guide to magical creatures. 

Because Beasts has no plot, it will serve as a jumping off point for the film trilogy. Rowling will write at least the first movie (and perhaps all of them) and be heavily involved in the production.

The Harry Potter books have sold over 400 million copies worldwide, according to Scholastic. Source: Scholastic. 

The hunger for Harry
The Potter books are sold in over 200 territories, have been translated into 68 languages, and have moved over 400 million copies worldwide, according to Scholastic.The eight movies have combined for over $7.7 billion in ticket sales with the final film in the series Harry Potter and the Deathly Hallows Part 2 grossing over $1.3 billion -- the most for any movie in the series, according to BoxOffice Mojo

Since the series ended, Potter's popularity has grown. The first generation that read the books are sharing them with their children and any little comment Rowling makes about her characters sends the Internet into hysterical speculation. In the stories she posted on Pottermore, there have been mentions of Harry, Hermione, Ron, and the rest which suggest they have further adventures, but for now, all fans are getting is the movies based on Fantastic Beasts.

But, Beasts, is not a Harry Potter book
however, is not a book about Potter. Because it's essentially a reference book, it's a blank canvas for Rowling and Warner Bros. The new trilogy could essentially be about anything within the world the author has created. Logically, since it's implied in the Potter series that the book has been around for a while, the films should be set before the events of the seven Potter books. And, while it's pretty much assumed that the trilogy won't be as directly linked to the Potter movies as the Star Wars prequels were to the original trilogy, they could answer some of the mysteries from the first series, and show how certain events came to be.

Rowling explained the general framework in a Facebook post.

Although it will be set in the worldwide community of witches and wizards where I was so happy for seventeen years, 'Fantastic Beasts and Where to Find Them' is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world. The laws and customs of the hidden magical society will be familiar to anyone who has read the Harry Potter books or seen the films, but Newt's story will start in New York, seventy years before Harry's gets under way.

In that way, the closest recent example would be The Hobbit movies, which made just over $1 billion (2012's An Unexpected Journey and just under $1 billion (2012's Desolation of Smaug), according to Box Office Mojo. Those are lower returns than the final Lords Of the Rings film Return of the King, but better than the first two movies in that series.

Though that series does share some characters with LOTR, much of the book (and films) sets the stage for the events in Rings. If Rowling can walk the same line, maybe even include some familiar peripheral characters, then she could capture the same box office magic.

Can it work?
The first film in the series is a can't miss blockbuster due to the same pent up demand that powered Star Wars: The Phantom Menace to $1 billion in global box office. But, if the movie is lousy (as Phantom Menace was) then it could fall of as the second Star Wars prequel did (Attack of the Clones took in a relatively scant $649 million).

Warner Bros. should be sitting on a trilogy of $1 billion box office hits, but in writing the movies, Rowling must deliver enough of the familiar to keep Potter fans interested in what are essentially Harry Potter books without Harry Potter. A new trilogy following the extended adventures of Potter and the gang would be a sure thing while Fantastic Beasts is one sure thing and two dangling question marks. 

It all depends on whether Rowling can weave her storytelling wizardry a second time.