The good news is that the core team is working hard on improving the state of documentation.
You have the Merb Book on which I am actively work on with a team of 20 people (writers, editors and translators).
You also have the documentation browsing application we are developing to let you browse documentation based on where you are in your app.
Finally, you also have other resources like the Merb peepcode.
The point being that we are actively working on improving the overall state of documentation. However, if you wish to learn more about the internals, how to some more advanced stuff like writing plugins, building your own stack, knowing when to use what component, I would suggest you attend the training class that Yehuda Katz (Merb lead developer) and myself are giving in Phoenix, AZ, January 19-21.
This would give you a rare opportunity to spend time with some of people directly involved with the day to day development of the framework as well as other like minded developers.