Basically is just Android, so the best is study Android system in all parts..what e-yes and Nitdroid team basically do is feed Android system with N900 hardware components drivers and tells the kernel to load them so the chapters are:
1) How Android system works
2) How to compile a driver in Linux and load it
3) Connect Nitdroid in Debug usb mode, load the driver into the system (insmod like) and see if it works..."works" means that the system handle with it without strange errors