It's very odd that neither Bryan A. Garner's "A Dictionary of Modern American Usage" nor Wilson Follett's "Modern American Usage" nor H.W. Fowler's "Modern English Usage" have an entry for the word "fundamentalist".
Yet it is such a peculiar word, and it gets tossed around so much nowadays. And lots of seemingly disparate groups of people get lumped together using this all-embracing word. For example, Joe Schmo, a top notch news reporter from Time Magazine, will handily use it to take a Bible-clinging Southern Baptist in Texas and somehow toss him in together with a shaheed in Syria who's wiring up his suicide belt. For Joe Schmo, somehow they're both considered "fundamentalists." How strange that this one word manages to embrangle so many different things. Yet nobody among the literati has tried to sort out all its nuances.
I suspect that this word has an interesting history and would be worth studying all on its own, from a philological point of view.