Thanks to a thread at the MobileRead.com forums, I’ve learned that you can add a menu item to the font-size/page-orientation menu accessed via the “Aa” button on the Kindle keyboard. This menu adds the ability to select between full justification (lines are spaced as needed to make both the left and right margins even) or left justification (no spacing is added, resulting in a “ragged right” margin). This is not a hack, but a built-in capability which for whatever reason the Amazon Kindle development team decided not to include in the default menu. (If anyone knows why, I’d like to know.)
In order to enable this menu option, you just need to edit a text file on your Kindle. (Note that you may need to enable the display of “hidden” files on your computer’s file system viewer before you can find this file.) UPDATE: Before connecting your Kindle to your PC, return your Kindle to the Home screen (i.e. close any book you’re currently reading), or else the following editing will not take effect. (Thanks Donna!) The file you want to edit is system/com.amazon.ebook.booklet.reader/reader.pref. (If you’re not particularly geeky, that means after connecting your Kindle to your PC via the USB cable, locate and open the system folder on the Kindle, then open the com.amazon.ebook.booklet.reader folder in there, which should then contain the reader.pref file.)
IMPORTANT: When you edit the reader.pref file, use a text editor (NOT a word processor) which is “UNIX line-ending aware.” (If you use Windows Notepad, it will insert Windows line-endings instead of the UNIX line-endings.) If not sure, I’d recommend using Notepad++, a free download (and not to be confused at all with the Windows Notepad editor). Also, be sure to make a backup copy of the reader.pref file somewhere, just in case you screw things up.
Once you’ve opened the reader.pref file in your text editor (a final reminder, don’t use Windows Notepad or Word!), just add this as a new line:
Now all that remains is to eject your Kindle from your PC, then restart it (Home -> Menu -> Settings -> Menu -> Restart). Once it has completed the restart and you’re back to the Home screen, open up a book, and verify that you now have the “Justification” item in the menu when you press the “Aa” button. Here are a couple screen shots from my Kindle 3 showing the same page with full and then left justification (click the images to see larger versions):