Which part are you interested in? A:M->GBA palleted sprites? Coding for the ARM7? The hell that its sound to GBA?
We basically searched for resource straight from google. The official NOA documentation is a big no no but there are some good groups out there that have basically reverse engineered the whole platform and opened it up to amateurs. First stop will probably be the HAM SDK which include an gcc arm compiler and some neato tools. I must warn you though, you either need to be a C++ coder or working VERY closely with one, this stuff isn't cake. There is no file system, which means all images/sounds/scripts must be converted to headers to be compiled directly into the flash. I love developing for it though, in fact the only reason we stopped in the first place is my buddy Guy took a job at Backbone Entertainment in Emeryville to do GBA programming (hence he can't do it outside of work). So if you are interested in doing a project, let me know! delaney@gsddevelopment.com.
P.S. Love your work man! I'm a big pinup fan.