Re: The audiobook released on May 8 but it didn’t download and won’t let me manually download it. The notice says the book isn’t available. The author moved it into kindle unlimited but I’d already bought it before she did that.
Maybe check whether your purchase is available to be "Re"downloaded:
How to: Redownload books and audiobooks - Apple Support
or
What to do: If there’s an issue when you redownload apps, movies, or other content from Apple - Apple Support
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Else if you want to request a refund:
See how to: Request a refund for apps or content that you bought from Apple - Apple Support
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If you want to reach out to Apple for more info. / support / contact options etc:
You can: Contact Apple Support - Apple Support