If someone uploads a book on a file-sharing site and you download it, would that contravene copyright laws? If it is, does it mean that borrowing a book from a friend or lending some of your books to fellow book-lovers is not permitted? If you obtained the book legally [by buying it] should anyone dictate to you how you use that book?
This is a very interesting question, though I think it is very black and white. First, A physical book has a single presence, and can only be shared once; something you cannot guarantee with a file. Second, generally, I believe that an ebook purchase is more like a licensing of the content than it is the purchase of a file. You'd have to look at that model to really understand why this is copyright infringement. In short, as far as I have heard, if you share a book this way you are liable for a lawsuit.
As jdwriter stated, it's a clear issue without any loopholes or confusion. If you own a physical book, then you actually own that book and can give it to other people. A digital copy, or e-book, is a license to view that material at your leisure. You can let someone borrow your e-reader or computer to read that book, but you're immediately breaking the law if you put it on P2P sharing sites. There are some stiff penalties for doing so, and the fine will be based upon how many people downloaded that book. The people downloading it are breaking the law, too.