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I'm not entirely sure of the answer here, it'll depend a good bit on what mode Waterstones uses to sell their books. If they use the same software that Sony has been using in the U.S. and Canada, then (for reasons known only to mysterious figures at Sony) it won't run (directly) on a MacPuter. However, it usually will
run under a Parallels or BootCamp installation of XP or Vista.
If that's the case, then you'll probably be able to go one of those paths. If it's not, well, it'll depend on what they're using instead of the Sony application.
That all only applies to encrypted BBeB, and Adobe books, howerver. The 505 shows up as a standard flash drive when connected to a MacPuter's USB port, so any un
encrypted books that you get elsewhere, which are one of the Sony supported formats, will work just fine being shuttled in through the USB port, just as you'd put files on a flash drive.
I hope that helps, please do let us know if you have other questions, or if I wasn't as clear about all that as I think/hope I was.