I'm an experienced developer which quite some experience in meta programming. I just found my way to Ruby and want to understand more of Rails (and related) project internals. Even if I understand the high level concepts, it's hard to get an overview of those projects. I wonder how other people approach that problem? How would you for example approach the Mongoid source code to get a good understanding of it's internals?
I hate to recommend another book, but I think this one pretty much has what you are looking for. Personally, I like the structure and completeness that books provide when getting into something that I'm unfamiliar with (which, consequently, is a lot of stuff)
If you are new to Rails, the best thing you can do is start from the beginning. Learn the framework and code up a couple of projects. This will get you accustomed to the "Rails way" of doing things.
Once you are comfortable with Rails, you can start diving in deeper to really understand how things work behind the scenes.
I recommend this book to start out. It's a little on the basic end, but it does provide a good starting off point in my opinion.