Here are several points to consider, none of which (except the last one) take a moral stand on anything to do with ebooks and sharing.
1. Calibre can save to file, send to device, etc. so in that sense, yes, it can be used to share books because it can turn the books into files that you can give to someone else.
2. You can also use the Content Server to share books over a network and even the internet.
3. Just because you purchased the book legally doesn't give you the legal right to share it. That all depends on the law in your jurisdiction.
4. Most books you purchase from the major retailers will have DRM (Digital Rights Management). Depending on the DRM scheme, merely giving someone a copy of the book file may not be sufficient to allow them to read it. Generally, if you can read the book in Calibre's built-in reader, it means it doesn't have DRM and it can therefore be shared just by giving them a copy of the file.
5. If a book has DRM, Calibre might be able to strip the DRM if you have the right plugin. Look up Apprentice Alf. That is all we're allowed to say on the matter in these forums.
6. DRM is EVIL!!!!!!!!!!! Sorry. I couldn't help myself. I just had to get that out of my system. This is just my opinion, of course.
