This chapter has provided a thorough description of Spring's AOP infrastructure. However, the following books and articles provide useful background on AOP and its applications:
J2EE without EJB. Chapter 8 is a pragmatic discussion of AOP in J2EE applications. The chapter discusses the motivation for the Spring AOP framework at length.
AspectJ in Action by Ramnivas Laddad (Manning, 2003). Excellent AspectJ text, which also explains AOP concepts very well.
Eclipse AspectJ: Aspect-Oriented Programming with AspectJ and the Eclipse AspectJ Development Tools by Adrian Colyer, AspectJ lead (Addison-Wesley, 2005).