According to ISO 2108:2005, it says "Each different format of an electronic publication (e.g. '.lit’, '.pdf’, '.html’, '.pdb’) that is published and made separately available shall be given a separate ISBN."
E-books from O'reilly have same ISBN for pdf, epub, mobi.
Additionally, my pdf book have print edition ISBN on copyright page.
O'Reilly ebooks are DRM-free so when you purchase an ebook you will get access to all the file formats available for that ebook. There would be only one ISBN for the ebook since you will get access to the different file formats under that ISBN. You may see the print ISBN on pdf books as the content for the pdf comes from the same source as the print version. More information on O'Reilly ebooks can be found here: http://shop.oreilly.com/category/cust.... Hope that helps and have a great day!
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