Via a link on Boing Boing, a post on Gizmodo about research on the ownership of content bought for e-readers such as the Kindle and Sony Reader. It brings up the issue that it would appear that you’re only licensing the content of the books, not buying them in the traditional sense of having outright ownership, with the associated the right to sell on and lend to others.
As the authors of the original research (access appears to be subscription only, but the Gizmodo post includes the article summary) conclude though, if it appears to be a sale, even if it calls itself a licence, it’ll be regarded as a sale.
But you couldn’t sell a copy of your document (some small thing called copyright!), you would have to sell the physical storage device the file was downloaded to. Or perhaps find a way of getting the downloaded file off the reader, leaving no trace / copy of it behind. And as someone points out in the comments, there’s no requirement that says the publishers have to make that process easy for the downloader….
The research is based on US law, and, not being a lawyer, I can’t comment on whether the “first sale” doctrine has an equivalent in the UK.