Map/Reduce is really hard. As you can tell, this thing takes the operational pain out of Hadoop... not the thinkin part :) Beyond something like this: http://hadoop.apache.org/core/docs/current/mapred_tutorial.h... there isn't too much in the way of docs on how to think in map/reduce that I'm aware of, but I found the classes in the Pig tutorial to be quite helpful.

edit: oh, guy above me has great link!