The Entropy-infused creatures designed by Steve Russell evoke the old school "negative energy" quasi-elementals, and apply the theme in an interesting and nasty way as a set of new templates.
In 4 packed pages of material, you get the rules to make ash-, dust-, salt-, or vacuum-infused creatures to wreck havoc on your party. Additionally, you get a fully designed creature with the dust-infused template applied, complete with roleplaying tips and a fleshed-out encounter. Finally, the preview presents two new feats which can be applied to any burrowing creature.
The preview features the gorgeous cover art by Hugo Solis (slightly obscured by the "free preview" text, which I would expect), and a fantastic illustration by Grey Thornberry of the frightening dust-infused feirling.
There are a couple of nit-picky issues which should be picked up in final editing ("entorpy" in one place for example), but not enough to detract from the rating.
In conclusion, who doesn't like free previews, especially for something you can put into your game right away?
[Disclaimer: I contributed a template to this book]