学习Rails代码库内部有哪些好资源？

I have been developing in Rails for about 3 years now, and would like to learn more about the Rails internals themselves. Rails 3 supposedly provides nice API's for integrating more closely with Rails, but I'm having a hard time finding documentation on how to use them. Here are some examples of what i'm looking for:I have been developing in Rails for about 3 yea