I found a very nice introduction to the assembly language of the processor in the Neo- the 68k. They talk about the sega megadrive, but the basic instructions should be the same.
I'm sure the Neo's processor has quite a lot of functionality beyond what it presented in this tutorial, but they cover the basic instructions. I believe the specific processor in the Neo is a later version with extra functionality, and I don't have a good reference yet for that model, or the hardware setup involved.
I am not exactly new to assembly, if you are then this tutorial attempts to help even the most beginner assembly programmer with the basic concepts, and walks through the basic concepts, and then each instruction. There are a lot of examples for each instruction. They discuss uses, edge cases- its a really helpful introduction. They even give homework to help you decode assembly listings.
I just wanted to point this tutorial out. It was clearly a lot of work, and I benefited from this work, so I want to point others to this resource.