For me, one of the most confusing things in Rails is routing. The DSL, all those automagic methods, abuse of method_missing (it's starting to feel usual), and RESTful conventions, mixed with namespacing,
I've found three detailed tutorials (well, series of tutorials) that seem to explain all what's needed to get into the routing thing. Here are the links to
A couple of articles on extending routing system
Here's the starting point of a super-detailed explanation and hands on tutorial on rails 2.X routing.
There are 6 parts in this last tutorial.
Btw, both blogs seem good sources of information. Good explanation on different topics, always quite detailed, and well written. For example, here's a series of posts on AJAX (from the bottom up).