Ficbot, ultimately fairuse is determined by precedent. For example, we know that timeshift (i.e. recording and rewatching video and/or audio broadcasts) is fair use because the Supreme Court said so in the 1980s. Likewise, I am reasonably sure that they have similarly announced the same for format shifting (i.e., converting from mobi to txt, html, etc). If your sister lent you her reader and you read a book on it, that is a clear example of fair use because it is no different than loaning a regular book. What you are describing however gets to be a rather murky area. As soon as it involves a device that is not your sister's, you enter a legally untested area. My own feeling is that you downloading copies of a work someone else paid for (unless it was paid for as a gift) does not constitute fair use. That being said, the only way to be sure is to get fictionwise or some other ebook publisher to sue you, get the case taken to the supreme court and see what happens
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Let us know how that goes in about 10 years why don't you
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Bill