I mentioned Mangle in my post!
This isn't a Mangle replacement... Mangle doesn't actually do any major filtering or compression of your images, aside from moving to greyscale and GIF (256 color GIFs as far as I can tell). Since the Kindle 2 is limited to 2GB of space, the Photoshop script/action cuts down file size significantly by reducing color count, without sacrificing much (if any) image quality. As far as I can tell, Mangle doesn't actually improve the contrast ratio or get rid of compression artifacts either.
Anyway, Mangle is totally great (I use it!)! This is NOT a total Mangle replacement, and you should STILL run your images through Mangle to get the book thingy, BUT, it doesn't do nearly the same thing this does!
Of course, you can also one shot any image with this too, for screensavers (waiting on that for K2) or the general image browser (which seems better this time around).
The newer version of Mangle also does image trimming and centering too, which is great!
*Note: If you want to drop file size down even more: get rid of the centering part of the action (the canvas resize part), and set your output to a 16 color .gif file, that'll cut things down even more without losing much quality (though it won't center your images on the Kindle anymore). If anyone wants that version, I can post it here too.