- http://gameboy.mongenel.com/dmg/gbspec.txt (html version http://problemkaputt.de/pandocs.htm#soundcontroller ) - http://www.otakunozoku.com/nintendo-gameboy-cribsheet/ - http://realboyemulator.files.wordpress.com/2013/01/gbcpuman.... - http://students.washington.edu/fidelp/galp/megaguides/GameBo... (The original Manual)
There are more, but I can't seem to find the links right now :O
You can also grab the RGASM project from the OtakuNoZoku website. I think Carsten (Surfsmurf) may have revived it on his github. I think I also pushed my source trees to github too. There is a GBC emulator that is both a COM component, and works inside of Visual Studio (at least, VS from around year 2000). Bunch of docs too kicking around. I have copies of most of them still.
I wrote programmer docs, assembler (before switching to extending RGBASM), linker, patcher, emulators, games, visual studio plugins, debugger, cartridge downloader, serial port driver, bunch of libraries, etc. Ah... the good old days.