I am currently developing some plugins for Redmine, and I must say, I'm a bit disappointed with the lack of documentation available on this subject.
Normally, If I am looking for explanations on classes/methods, I need to spend a consistent amount of time browsing trough the core classes, and striving to understand the code written there (It seems that some people forgot to document their code, unfortunately).
I've checked their forums http://www.redmine.org/projects/redmine/boards, and it seems to take forever in order for a question to be answered.
That's why I would like some advice regarding some good API documentation/books/forums on this subject.
the better documentation is the codesource. One the Redmine's mainteners have almost all Redmine's plugin on github.
you can choose any plugin and see what is looks like. mostly it's about metaprogramming and alias_method_in_chain usage