Because of all the Spring 3.x books out there Craig writes the most extensive, exhaustive, and easy to read books about Spring. In many ways you can consider it the boble of Spring. Craig has a great way of explaining the topics. For instance, I use his analogy of a SuperHero getting its SuperPowers by external sources rather than the SuperHero creating them themselves for describing Dependency Injection, when I teach the Core Spring class.
On some topics, I feel Craig does a better job at explaining how to use it than the official Spring documentation. Spring Security is a great example, where it takes a few reads on the Spring Documentation to grasp it, whereas one read on Craig's chapter is enough.