[FizzyWater]

I don't think Amazon or any other ebook publishers maintain separate copies of each book sold and ties those to individual purchasers.

If a book is sold without any kind of digital rights (DRM), I'd say Amazon/whoever provides a link with the ability to download that one copy. NOTE: If you mean here that at this point there are now 2 copies - one on Amazon's servers and one on the customers, well, then, yes. Only way to avoid that is to stream and not allow offline downloads.

If a book is sold with DRM, then to some degree it depends on the version of DRM. Certain watermarking DRM would take the raw copy of the book and add additional code to make it unique to the buyer. If the buyer were to download another copy, it would again go through that process - the seller wouldn't keep a copy of that processed book. As an example, my old Palm Digital Media/eBook PDB ebooks included my name in the copies.

The party line is that DRM prevents customers from passing on copies. But most DRM schemes can be gotten around (and for the truly determined, anything that displays on a computer screen can be OCR'd with the right software).

What prevents a customer from giving it away is their own honor (and perhaps level of technical interest in learning how to get around DRM).

NOTE: I recently read a book by an author who included included a scene where one of her characters shares his ebook reader with another character, so they could talk about the book after character 2 read it. I absolutely loved that the author acknowledged that ebook readers want to be able to do what they did with print books - share them with friends who are book lovers.