See if your library has this book:
http://www.amazon.com/Computer-Organization-Design-Fourth-Edition/dp/0123744938
It's what we used in my computer organization course and I found it to be quite helpful. If you are desperate, a PDF version of your book is comes up as the second result on google...
There are a few ways. One is to treat the assembly language like just another programming language, just with some really weird rules. Another is to start from the bottom; start with learning instruction set architectures in general, and MIPS specifically. Having already studied a few CPUs in school and on my own, I'll often start by looking at Wikipedia, then finding some kind of official documentation, but that's going to be tough, without being taught some of the basic ideas first.
Looking at OSSU, I'd guess that these courses would have appropriate information (I don't know how deep you want to get into it, and it's probably overkill):
http://www.cs.cmu.edu/afs/cs.cmu.edu/academic/class/15213-f15/www/
https://inst.eecs.berkeley.edu/~cs61c/fa14/
https://www.coursera.org/learn/comparch
Patterson & Hennessy's Computer Organization and Design can be bought used for under $10. It specifically covers MIPS. I've got the 3rd edition on my bookshelf.
Or...just jump on a thread, and start pulling. Wikipedia's article on Instruction Set Architectures probably isn't a terrible place to start...although that's just the very beginning (and has information that won't matter to you mixed into it). I feel like one of the core aspects of reverse engineering, especially when you're working on specific games, is that no one's going to tell you the answer. You've got to get good at finding information and digging on your own. Do research (that doesn't always pan out), find experiments to try (even if the information ends up being useless). You'll find the patterns with enough effort.
This Computer Organization and Design? http://www.amazon.com/Computer-Organization-Design-Fourth-Architecture/dp/0123744938
Some of the reviews are scaring me dude...