http://www.comms.scitech.susx.ac.uk/fft/programming/R4400_Uman_book_Ed2.pdf
You will not need to read the whole text book, but it will come in very hand if you are struggling with MIPS. Choose what you read out of this text book.
I found it at the following site http://2020ok.com/. Here you may find some other useful books on a range of other computer related topics.
Also check out http://en.wikipedia.org/wiki/MIPS_architecture to get a better understanding of MIPS.
The official web site for spim is one of the best places to go and has a lot of extra resources, make user you visit this web site. http://pages.cs.wisc.edu/~larus/spim.html.At the bottom of the page is a section on additional materials.Lastly here is a link to a document I got of the net and cant remember where. I found it gave a good overview of all the very basics and is an excellent reference when writing simple MIPS programs. http://www.scribd.com/doc/2576056/SPIM
