How can I left justify text on my Kindle?

The justification is normally specified in the CSS included in your ebook. Commercial ebooks often come 'justified', just like most traditional books (which generate less of a problem of large white space, through the combination of hyphenation and wider lines).

It might be that the Kindle overrides the ebooks default, but I doubt it. What you should try is change the CSS in an ebook, e.g. by convert the ebook with Calibre, while specifying Text Justification: Left Alignin the Look & Feel tab of the book conversion page.

Calibre has several add-ons from the shadier parts of the internet which strip various kinds of DRM easy as pie. Just Google "Calibre add-on strip kindle drm", install it, Lisa's your books from you kindle to your PC and it strips then in the background. Technically it violates the end user agreement but you get to left justify your text and since your not pirating the books in the first place you run no chance of displacing potentially sustenative income for the authors.

The problem can be mitigated slightly by using landscape rather than portrait mode on the hardware Kindle, and longer line settings in various Kindle apps. Higher average # of characters per line with these settings means two things.

1. Lower number of line breaks per book, therefore lower expected number of awkward ones.

2. Larger average number of spaces per line over which to spread the space needed for justification.

Again, this is a slight mitigation of the problem, it is far from anything that could be dignified by calling it a solution to the problem.

And, depending on your preferences, it may trade off one problem (high probability of awkward line breaks) for a worse one (unpleasantly long lines).

This is one of those problems with Kindle that is shocking to me but doesn't seem to bother many people. Kindle is an amazing device, particularly from a hardware perspective (resolution, battery life, built-in light) but it is uneven, suffering from what seem to be easy-to-fix problems like the justification butchery you mention. It seems like they got many of the hard things right and some of the easy things wrong. I particularly resent the beautiful full-page Kindle ads I see, for example, in the NY Times Book Review. It is probably not true, but I can't help thinking: if only they put the same kind of effort and visual acumen into the device's software that they put into the device's ads...

Another possible option for books purchased from Amazon is to see if there's a newer version of the book that's using their newer KFX format, what I think they call enhanced typesetting. In that case the Kindle's font/layout options do allow you to switch between ragged right and fully justified. Otherwise your only option is to remove the drm which allows you to edit the CSS in Calibre.

Yes, you can. In a modern Kindle device you can select the formatting and left-justify it:

But this feature is just available for books you purchased from Amazon. Some older books don't have this feature, but I believe they are reprocessing everything. You can check in the book details if enhanced typesetting Is activated.

You can't do it with your own uploaded personal documents.

If you really want it, you can generate a Kindle book with enhanced typesetting using Calibre.