KindleGen

Creating e-books that can be made available for purchase through one of the most well-known online platforms may sound like a daunting task. However, if you get a hold of the appropriate software, the whole process can be actually very simple.
For those who prefer a 'quick-and-dirty' tool that can get the job done without going through several windows, tabs and complex configurations, KindleGen might be exactly what you're looking for.
This particular application doesn't even have a GUI and can be deployed only via command-line, but this should not be an impediment for the more advanced users. Beginners, on the other hand, may be daunted by taking on such a challenge, at least until they realize that when opening KindleGen everything is explained quite clearly.
This utility is able to handle a variety of formats for input documents, including HTML, EPUB and OPF. It can even handle directories and ZIP archives as source, but only for ever-eXtending Mobile Document Format e-books or XMDF, in short, and FictionBook files (FB2).
With KindleGen the e-books can be created with standard DOC or Kindle huffdic compression, but, just as well, they can remain uncompressed if necessary. In case you want to get as much feedback as possible during the actual conversion, you can add the 'verbose' parameter to the string of commands.
A nice feature of this app is the ability to have all the pictures in the book converted to GIF images, which will save some space and help keep the overall size of the e-book as low as possible. Also, if you want the messages to be displayed in other languages, the localization function of this tool makes this possible.
The conclusion is that the only way to appreciate what KindleGen is all about is to put it to the test. The processing is done quite fast and the results are as accurate as the input allows it.