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Right after checkout, Gumroad emails you a receipt with a link to your downloads: the book (PDF), the phone edition (PDF) and the audiobook (MP3). You can download them again any time from that email or from your Gumroad library.
On an iPhone, open the PDFs in Books; on Android, in Google Drive or any PDF reader; on a computer, in any PDF reader. The audiobook plays in any music or podcast app.