I managed to have my left-aligned book inside my Kindle, but I warn you that was way harder than I thought.
You must convert your book to the Amazon proprietary and undocumented format, KFX. This format will allow you to have an ebook with Enhanced Typesetting. Just books with Enhanced Typesetting can be left-aligned.
Now here are the instructions to have your left-aligned book in a dozen complicated steps. Next year, Amazon will probably have changed its tools and this info will be outdated.
- Install the wonderful Calibre
- Install Amazon Kindle Previewer 3. It should be installed in the default location under the same user account as is being used to run calibre.
- 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.
- All the magic is made by the KFX Output plugin, here is KFX plugin documentation. Any problem you have in this process, the answer is probably in this thread.
- Now I'm considering that your book is already added to your Calibre library. This is a good moment to check and fix the book metadata.
- Right-click your book and select "convert individually"
- Next dialog, top right, select KFX, run and wait for it to finish.
- Now connect your Kindle to your computer using a USB Cable. No, you can't email your file. You can't use "send to kindle". You must transfer it using a cable.
- Right-click your book, select send to device, send the specific format to device, select KFX and send it.
- Eject the device.
Now you have your left-aligned book in your Kindle. I don't know if it is worth the trouble.
If you need to check if your book is correctly converted, rename its extension from kfx to awz8, open it with Kindle Previewer, got To View menu, Book Information and it must have "Enhanced Typesetting" supported: