This will probably get asked or pointed out, but there's a Apple iBook FAQ at:
http://support.apple.com/kb/HT4059
and for right now, at least, you can only buy an iBook if you're on a limited set of Apple platforms:
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Can I purchase a book from the iBookstore using my computer?
No, the iBookstore is only available through iBooks on iPad, iPhone, and iPod touch at this time.
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You can view it on up to 5 computers, etc., the FAQ is useful.
But yes, if I don't buy an iPad, for now at least, I'd be blocked from the iBookstore. Not a big factor in my deliberations, but I suppose it might be for others. What is a factor is if I can Read the book from multiple providers (Amazon, B&N, etc.)