Now you have your left-aligned book in your Kindle. Right-click your book, select send to device, send the specific format to device, select KFX and send it.Now connect your Kindle to your computer using a USB Cable.

Next dialog, top right, select KFX, run and wait for it to finish.Right-click your book and select 'convert individually'.This is a good moment to check and fix the book metadata. Now I'm considering that your book is already added to your Calibre library.Any problem you have in this process, the answer is probably in this thread. All the magic is made by the KFX Output plugin, here is KFX plugin documentation.In Calibre go to Preferences (the icon may be outside the ribbon) -> Add plugins -> write KFX in the text box -> Select 'KFX Output' plugin, right click on it, and install.It should be installed in the default location under the same user account as is being used to run calibre. Next year, Amazon will probably have changed its tools and this info will be outdated. Now here are the instructions to have your left-aligned book in a dozen complicated steps. Just books with Enhanced Typesetting can be left-aligned. This format will allow you to have an ebook with Enhanced Typesetting. You must convert your book to the Amazon proprietary and undocumented format, KFX. To test out the ability to create Kindle files, open the Lena sample from Browse Samples on the Help menu (or open your own project), select the Kindle configuration in the Control Panel, and press Compile. I managed to have my left-aligned book inside my Kindle, but I warn you that was way harder than I thought. Select Use Kindlegen from Kindle Previewer 3 and then click Install.

Kindle previewer