http://shop.ironcrown.com/index.php?main_page=index&cPath=7_53
HARP Sci-Fi has been out of playtest for a while.
The GenCon version (a physical book) was a quickly pagemade version of the manuscript.
The Gamma version (PDF only) has corrections made and typos fixed and is the most recent version. It is final except for artwork.
The way that ICE's online store works, once you purchase a PDF, you can download that PDF from your account at any time. And the Gamma pdf is set so that once we get the final artwork, we can just replace the file on the server and change the name of the entry - thus all those who buy the Gamma can then just download the final when it is available.