We are excited about the upcoming "Gradle in Action" (Manning) by Benjamin Muschko, long term Gradle contributor and author of numerous Gradle plugins. We've reviewed the first chapters and they look great!
There is now an early access version available. Manning provided us with the promo code gradlelaunchwa that saves you 50% on Gradle in Action. The code is active until Feb 3, 2013 at 12 midnight.