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The new Harry Potter has become the biggest book of the summer isn’t a blockbuster thriller or a splashy celebrity memoir. It’s a play about a middle-aged wizard.

The publication of “Harry Potter and the Cursed Child,” the eighth installment in J. K. Rowling’s best-selling series, is being celebrated at more than 5,000 bookstores and libraries across the country this weekend, with midnight release parties featuring costume contests, magic shows, wizard rock bands, live owls and butterbeer, the beverage of choice among young wizards.

The elaborate rollout has all the flourishes that fans have come to expect for a new Harry Potter book. But for many nostalgic readers, this one feels different. “Cursed Child” is not a new novel, but a script of a play — a format that typically isn’t read for pleasure and almost never produces overnight best sellers. And unlike the previous seven books in the series, it was not written by Ms. Rowling herself.

In a sense, “Cursed Child” is more like sanctioned fan fiction than a new work by a beloved writer. Ms. Rowling worked on the play’s plot with the playwright Jack Thorne and the director John Tiffany, and while she helped shape the story, she has made it abundantly clear that she did not write the script.

The idea for the play, which explores Harry’s life as an adult and parent, didn’t originate with Ms. Rowling, either: She merely agreed to it when two theater producers proposed the concept.
Which raises a sticky question: If Ms. Rowling didn’t conceive the play, or write it, is “Cursed Child” really a new Harry Potter story by J. K. Rowling? And if it isn’t, do Harry Potter fans even care?

Many, apparently, do not.

“J. K. Rowling’s involvement legitimizes it as canon, and the fact that other people collaborated on it doesn’t detract from that,” said Matt Maggiacomo, executive director of the Harry Potter Alliance, a nonprofit group that promotes literacy and other causes.