Understanding Rails core source code?

My advice would be to read this one: http://pragprog.com/titles/rails3/agile-web-development-with-rails-third-edition to get a pretty much complete understanding of how rails works on the outside, if you haven't already.

And then checkout the latest trunk and start reading a component you like, messing with files as you please and trying out the changes on a live project.

Metaprogramming is a concept used a lot in Rails, so this book would definitely be good too: http://pragprog.com/titles/ppmetr/metaprogramming-ruby.

Hope that'll put you on your way

maybe the book ruby-for-rails from David Black at Manning could help you as well

I've written the beginnings of an initialization guide for Rails 3 that may help you understand some of the common concepts in Rails 3. This covers mostly the "railties" part, but branches out in the actual Railties. It really depends on what you want to patch/look at in Rails as to how much this actually applies to you.