I haven't read this book of Marc's, but his Owning Rails course is excellent. It's definitely not for beginners, but is great if you're at least intermediate in Rails.
In the course he walks you through how to make a simple clone of the Rails framework. Once you've done that, digging into the Rails source isn't so intimidating.
In the course he walks you through how to make a simple clone of the Rails framework. Once you've done that, digging into the Rails source isn't so intimidating.
http://owningrails.com/