“Prepare the text by reducing it to single-space. And justify the text. Nothing screams amateur like ragged-right text.”
That seemed like good advice, but as with any advice it pays to check. So I put up the poll you can see at the top of this post. On first glance it seems that the quote is right, however the right answer is actually a little more complicated. I recommend reading through the post if you are interested in find out what experienced ereaders want (and if you’re publishing on Smashwords it’s a must).
In a nutshell if you are publishing through DTP at Amazon then go with fully justified. If you’re going through Smashwords leave it as left justified, or don’t specify, because most other ereaders (excluding Kindle) will reformat according to the User settings.