If you don't want to read it, I'd suggest coming up with a small, simple project - just build a simple web application and try to add security to it. Use a step-by-step approach, first adding Spring Security without doing much else. Next, you can use the concept of progressive enhancement to continue to explore features of the framework once you understand the baseline. Spring Security also comes with some good sample applications, and a well-written reference manual.
Actually, for most of the simple functionality, you don't need to get into the source code at all - you can do a lot simply with the XML-based namespace configuration, which abstracts a lot of the back-end functionality.
Hope that answers your question (and hopefully you win the book )
Author, Spring Security 3 (the Book), Packt Publishing, 2010