It seems to me that you are introduced by the thought and the genius of this technique as well as the knowledge of the basic elements and the rules of resonance of the latter. Besides that when studying the fuzzy expert system you will see that they can have heuristic and metaheuristic bases.
For more details information about this subject, I suggest you to see links and attaché files on topic.
If you want to review papers and research on Fuzzy Expert System, it is difficult to suggest to what point you should study on the same, without fully knowing how much you know about the topic. But generally speaking, you must know the fuzzy set theory and its applications very well (to an intermediate level, at least).
But I will suggest you that start with the basics, and after you are done with it, start going through the papers. And if you face problems while reading papers do your research topic-wise and try to return to the basics, in that way I believe you will have a better grasp of the topics and more. That being said, I am suggesting you some reading materials:
It depends on what you are after or what you trying to achieve. If you are reviewing papers this can be based on thematic/topical/subject or historical say from 1995-2000. However, if you are carrying out research, the deeper you go the more interesting things you might discover and the more you will know in that field. Such can make you an expert or a professor in that area.