Throughout the book we recommend further reading that will help you get a deeper grasp of concepts important to Spring development, such as Aspect-Oriented programming (AOP).
You may find it helpful to read J2EE without EJB (Johnson/Hoeller, Wrox, 2004), which provides a detailed discussion of the architectural rationale for lightweight containers such as Spring. However, this book is not purely a sequel to that book and can be understood entirely on its own.
There are many Spring Framework resources online. You should find the following particularly helpful:
The Spring Home page (www.springframework.org): Portal for most Spring-related information, including the reference documentation and downloads.
Spring Forums (forum.springframework.org): The place to go to ask questions about Spring. The Spring community is generally very welcoming and helpful.