Rowling quoted in a BBC article:

Rowling said, “the first distant rumblings of the weirdness that usually precedes a Harry Potter publication can be heard on the horizon”.

“Some, perhaps, will read this and take the view that all publicity is good publicity, that spoilers are part of hype, and that I am trying to protect sales rather than my readership,” she continued.

“However, spoilers won’t stop people buying the book – they never have. All it will do is diminish their pleasure.”

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