I think it is based on what you want to learn....are u interested to know the architecture of Processor then the link that I sheared will be good starting point....but if you want to say I want to learn the coding for the there arm based micro-controller is different ball game all together ....from how to code to how to optimize the code for the given processor core....any ways there is lot more than these ....if you want to see....my thought will be take any reference board like LPC2148 or some thing like and try to code for the functional aspect of system will make you bit more clear....
Good Luck