I'm starting development for the GBA with Jonathan S. Harbour's book and the HAM SDK. Jonathan's website links to the HAM 2.80 SDK, but I have read many reports that this version of the SDK does not compile C++ code properly and that one should use the HAM 2.71 SDK for Windows. It was quite difficult to find the HAM 2.71 SDK for Windows because nobody seemed to have it and the original website, www.ngine.de, has been down for quite some time. This is not surprising as the peek of GBA development was over 10 years ago. However, some of us are nostalgic for the old GBA especially if we can manage to get ahold of a flash cart like the EZ Flash IV, EZ Flash Omega, or GBA EverDrive. Provided below is a link to the Windows version of the HAM 2.71 SDK:
HAM 2.7 and HAM 2.71 Update
HAM 2.80