This is a good introduction to ruby's method lookup, even if it isn't fully accurate. The author notes that he hasn't read the ruby source code, and his explanation is "just a model you can use to understand things."

While mostly accurate, his description of the module hierachy as a tree leads to a wrong understanding of how ruby's method lookup actually works. If ruby did use a tree, then including module B in module A after including module A in class C would result B being one of C's ancestors, which isn't the case. Ruby's method lookup uses a linked list (not a tree) using iclasses for modules (a pseudo-copy of the module, which is why later includes have no effect). However, a description of that may be too in-depth for an introduction.