I have not read these novels. However, the difference is the same as the difference between "extreme" and "fundamental". A fundamentalist of X wants to stick to the foundation (probably the earliest form) of whatever X is. Examples of fundamentalists seen typically are people who subscribe to a religion (I think especially very old rules within a religion) and follow the rules very strictly. But I think this could be applied to fields other than religion. An extremist takes X to an extreme level, even if it's not about sticking to the foundation of anything. For example, I think there can be an extremist anarchist. He doesn't believe in any government or rules, and takes extreme actions against governments and rules. But he probably wouldn't be considered fundamentalist, for if he did that would mean there is a set of rules for anarchists to follow, and that goes against anarchist ideology, at least as I understand it.

One big difference between the words is that “extremism” usually refers to a person’s willingness to take ACTION (including violent action) in support of an extreme/fringe/fundamentalist beliefs, while “fundamentalism” refers just to BELIEFS. The fundamentalist beliefs may or may not be viewed as “extreme” — this depends on the perspective of who is judging. Also, you can be an extremist without being a fundamentalist — for example, “white power” activists in the US are viewed as extremist, but their views are not rooted in fundamentalist religious beliefs. Here are some examples of use (from the perspective of current-day American usage): 1. Many people in the West are concerned about the threat of violence posed by Islamic extremists. 2. He is a fundamentalist in her religious views, but he does not approve of any acts of violence to advance those views. 3. Many people view Christian fundamentalism as a threat to personal liberty and social progress in America. 4. The Saudi madrasas teach fundamentalist views, but only a few of the students become extremists. 5. Violent “white power” extremists are responsible for numerous recent hate crimes.