Whatever the fate of Harry Potter in his final book, one thing seemed sure: There would be a parting of the ways from J.K. Rowling.

Now, however, it appears that the seventh volume in the series may not spell the end of their relationship after all.

The multimillionaire writer is understood to be preparing for one last look at the world of Harry, Hermione, Dumbledore, and Voldemort after the publication of “Harry Potter and the Deathly Hallows” in July.

Using background notes from the past decade, Ms. Rowling, 41, is set to compile an encyclopedia of magic to create the ultimate guide to the world of wizardry.

It would cover figures such as the founders of the Hogwarts school, along with elaborate genealogies of the main characters.

It would also provide a chance to flesh out other figures who merit only a passing mention in the books.

In March, Ms. Rowling described her sadness at reaching the end of the series.

“I always knew that Harry’s story would end with the seventh book, but saying goodbye has been just as hard as I always knew it would be,” she wrote on her Web site.

“I’ve never felt such a mixture of extreme emotions in my life, never dreamed I could feel simultaneously heartbroken and euphoric.”

But suggestions of the plan have now appeared on her Web site, JKRowling.com.

Despite telling fans that it was “highly unlikely” that she would write any more Harry Potter novels, she wrote: “I might do an eighth book for charity, a kind of encyclopedia of the world so that I could use all the extra material that’s not in the books.”

The tantalizing possibility has been confirmed by her agent, Christopher Little, who said Ms. Rowling had “retained all the rights in the Harry Potter series” and pointed out that this included any companion books “which she may indeed write herself.”

The prospect of an authorized Harry Potter encyclopedia will have fans and booksellers salivating. Ms. Rowling’s notes are believed to be almost as detailed as those made by J.R.R. Tolkien for his fantasy Middle Earth, the setting of “The Hobbit” and his “The Lord of the Rings” trilogy.

Sales of the encyclopedia would almost certainly rival those of “Deathly Hallows,” which has advance sales of 1.5 million on the Internet store Amazon alone. It is tipped to break publishing records when it goes on worldwide sale on July 21.

An industry of Harry Potter books already exists. In addition to the seven books by Ms. Rowling, there are estimated to be about 190 Harry Potter-related titles cashing in on the success of the series, including “What Will Happen in Book 7?,” a book of predictions from the Harry Potter fan site MuggleNet.com, which has sold 300,000 copies in America. Other titles include a business book called “If Harry Potter Ran General Electric” and “Harry Potter and the Torah,” which offers a Jewish perspective.

In 2001, Ms. Rowling wrote two slim companion Hogwarts schoolbooks. “Quidditch Through the Ages” and “Fantastic Beasts and Where to Find Them” have raised more than $30 million for Comic Relief.

George Beahm, an American author who has written the bestselling “Muggles and Magic: An Unofficial Guide to J.K. Rowling and the Harry Potter Phenomenon,” said, “I certainly think there is room for more books given the place of the Harry Potter series in the fictional universe.”

“For my part, I’ve made no great riches — but it’s been a lot of fun,” he added.

Ms. Rowling, who as a penniless single mother living in Edinburgh in the early 1990s struggled to find a publisher for the first book, has seen her life transformed by the Harry Potter phenomenon. Now the richest woman in Britain with an estimated fortune of about $1.1 billion, she is married to Neil Murray, a doctor.

They have two young children in addition to Ms. Rowling’s daughter from her first marriage to Jorge Arantes, a Portuguese television journalist. Meanwhile, her writing talents have helped to create one of the world’s most successful brands.

With reports that the author is in negotiations to build a Harry Potter theme park in Florida, the bespectacled boy wizard now rivals Disney’s Mickey Mouse as a corporate cash machine.

The fifth film in the series, “Harry Potter and the Order of the Phoenix,” starring Daniel Radcliffe as Harry, opens on July 13 and is expected to rival newly released “Spider-Man 3” for the biggest box office opening weekend in history.