Full justification of a page — so the page margins are flush to both the left and right edges — sounds like what you want in a professional book, but when computers are laying out pages on the fly on small screens, and especially when they are under the constraints of having relatively few words per line to play with, it can result in ugliness.
When I first got my Kindle 1, it let you decide whether you wanted left justification (= “ragged right”) or full justification. Then Amazon upgraded the software and took away that option, which was not my favorite upgrade ever. (Maybe I just failed to find the hack to restore it.) I just got a Kindle 3, on the occasion of my Kindle 1’s screen losing a valiant battle against pressure in an over-stuffed backpack. There is a hack for the Kindle 3 that has restored the option, except where publishers have explicitly created fully justified texts. Go here and follow the advice in reply #1 scrupulously. (If you don’t know about UNIX line endings, you might not want to try this.)
I also altered one of the existing lines to “JUSTIFICATION=left”, which may be having the effect of setting the default to ragged right, but I’m not sure. At least it didn’t obviously break my Kindle. (Which reminds me: You’re responsible for whatever damage following the advice here may cause. What are you doing following advice in a blog, anyway?)