You Can Buy Purple Cow here or download it for free. He writes good stuff primarily about the new rules of marketing.

Cows, after you've seen one, or two, or ten, are boring. A Purple Cow, though...now that would be something. Purple Cow describes something phenomenal, something counterintuitive and exciting and flat out unbelievable. Every day, consumers come face to face with a lot of boring stuff-a lot of brown cows-but you can bet they won't forget a Purple Cow. And it's not a marketing function that you can slap on to your product or service. Purple Cow is inherent. It's built right in, or it's not there. Period.

In Purple Cow, Seth Godin urges you to put a Purple Cow into everything you build, and everything you do, to create something truly noticeable. It's a manifesto for marketers who want to help create products that are worth marketing in the first place.