What is the best way to learn from rails source code?

One great starting point is ActiveSupport, specifically the core extensions:

http://github.com/rails/rails/tree/master/activesupport/lib/...

Though this library isn't really Rails-specific, it has a bunch of self-contained ruby files, good to learn about the style of doing things elsewhere in rails.

Examples: Array#uniq_by, String#pluralize, Numeric#hours, etc.

The Rails router, while considerably more complex than ActiveSupport, is also a self-contained chunk of code you can read: http://github.com/rails/rails/tree/master/actionpack/lib/act...

Also: When reading Rails code (or any ruby library's code, for that matter), I've found that reading the specs first is very effective.

At the first line perhaps. Seriously, it's not that hard. You can also look at the source of methods at http://api.rubyonrails.com

Gregg Pollack's talk at http://confreaks.net/videos/285-lsrc2010-decyphering-yehuda is fairly accessible and gives a good overview of some of the newer refactorings.

Learn that 70+ loc method definitions are a terrible thing, and never do it in your own code :)