Hey there!
I have been using Calibre to convert from AZW to ePub for years, so that I can use the files with other software, like VoiceDream Reader. Using guidelines from this forum, I have been able to successfully force left justification by going into Look & Feel, Text, Text Justification, Left, AND by putting the following command under "Extra CSS" on the Styling tab.
body {text-align: left;}
p {text-align: left;}
So far I have to apply these settings every time for each newly imported book, and I am trying to figure out how to make the changes universal, so that it just happens every time. I went into the Calibre Preferences, and under Conversion, "Common Options" I made the two above changes. But still when I do a conversion, I have to apply the changes manually.
