Most Kindle ebooks have DRM (are encrypted) and this one is no exception.
You can buy the Kindle ebook by first registering a Kindle desktop app (K4PC or K4MAC). Calibre can only convert the format (MOBI to ePub) of DRM-free ebooks, but it does this very well. There is a 3rd party plugin to Calibre that can strip the DRM from Kindle ebooks (e.g. from K4PC or K4MAC). See Apprentice Alf's Blog for everything you might want to know about DRM.
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