GREAT LINKS: Programming The Nintendo Game Boy Advance. A free downloadable PDF e-book by Jonathan S. Harbour.
This book was not released in book stores because Nintendo put up a legal fuss... so Jonathan S. Harbour puts it on the web for free. Ha.
This book has been out forever, but I'm rediscovering it now because we now know that you can run GBA code on both screens of the DS - so you beginner homebrew game designers have quite a breadth of capability with this book and a little gumption.