Searching for docs and sources didn't yield a whole lot of results, but I think the following links will be interesting.
MP3' Tech - MPEG source codes Has multiple mp3 decoder sources, amongst other things.
MP3 - Hydrogenaudio Knowledgebase Looks like a nice page with some potentially interesting links, including one that describes huffman decoding.
www.eecs.umich.edu/~accheng/doc/MP3Decoder_AllenCheng.pdf Seems to be an in-depth description of mp3 decoding.
www.mp3-tech.org/programmer/docs/fpga_report.pdf Also a more in-depth description. It's a hardware implementation, but that shouldn't matter too much.
These are the best I could find without searching for hours, and although I'm sure they'll make for some interesting reading, I hope some of it is actually of some use to you. If not, I'm going to have to dig a 'little' deeper
