I suggest starting with the some of the FAQ's and literature where they try to convince you to use it. Once you understand their philosophy, learning the concepts will be easy because you will already have a guess at how they implement a feature.
After this point you must learn about Beans, ApplicationContext and configuration. Most other topics will require a basic knowledge of loading and configuring beans. Head to the (excellent) documentation, browse that for some interesting sections. If there is an api in particular you want to learn within Spring, tackle that after you "get" bean configuration.
Beyond that, use some reflection, why do you want to learn Spring? What will you use it for? Find something cool and try it out.
Spring works with Inversion of Control IoC and Aspect Oriented Programming (AOP). It also has a web tier, Spring MVC, along with may other aspects of the framework. This is the latest doc from spring, but I would start here and move off to books that show implementations of these ideas. http://static.springframework.org/spring/docs/2.5.x/reference/index.html I think springs docs are pretty good without the other books.