". . .Calibre allows you to convert content from various internet sources such as Project Gutenberg into the appropriate format for your e-reader, whether it is a Kindle, a Nook, a Sony, or something else."
What I find in practice is: Calibre is very slow and takes a fair amount of work to use in practice. It converts the input file format to its own internal format, and then to the designated output file format, and in the process seems to apply a bunch of heuristics and assumptions which don't seem to work out very well in practice for me. For example if I send a large set of Unicode code points into it, a smaller set of Unicode code points comes back out of it -- which I don't understand. I would have thought that "Unicode is Unicode" and that Calibre would pass it through unmolested. It crashes or hangs for me on very big and complicated stuff. Not to imply that it doesn't work more-or-less on simple stuff, and many people are using it happily. (I used Calibre a lot to "check things out" but have decided I can't rely on it for "checking things out" because it introduces a lot of its own sets of bugs and assumptions while making file format transformations.)