The Mothman Prophecies has been one of my favorite books for a long time. Stu mentioned how Fortean books wouldn't be Fortean if they were verifiable, and how Keel sought the "Big Fish" of a great story just as much as he sought out the facts.
Personally, I think this idea is a hallmark of Forteanism and that is why I fell in love with the school of thought. i think Keel has remained so prominent because he embodied this and encapsulated it in his work very well. I think this is also extremely recognizable in Fort's own work. isn't there a quote from Fort that goes something like, ""I believe nothing of my own that I have ever written." ? It's the idea that Forteanism is like a superposition of fiction onto non-fiction realism. the ability to acquiesce knowledge is on a sliding-scale, it just depends on the particular philosophical "vantage point", so to speak. kind of touches on the difference between a literal truth and metaphorical truth.
as far as Keel goes, I think he's one of the best writers I have ever read for his ability to craft non-fiction research into such a tantalizing, pulp-fiction-esque narrative. He made his books read like old school science fiction movies/books, and that was kind of the beauty in it.
Granted, that's all just my opinion in the long run.
I also really like researcher Jim Keith, did lots of Fortean research before he died. In his Coast to Coast Interview of 1997, Keith talked about Keel at some length when asked about Mothman, etc. He noted that while some of the phenomena defies total explanation today--there seemed to be a lot more government activity in Point Pleasant than Keel wrote about in his book. like Keel was searching for more of the Big Fish in the story instead of making it a book about the government. for that matter, he didn't deny any government activity in Point Pleasant, he just didn't focus on it. but it's worth noting.
furthermore, Keith said that he read about Keel's brief time spent in the military as a young man. He said that Keel actually once was given the opportunity in the military to run a War-of-the-Worlds-style radio broadcast, which was meant to test the boundaries of the audience and gauge the reaction of people should they be lead to believe it was true. But instead of War of the Worlds, Keel did a broadcast about Frankenstein! this theory kind of implies that the old war of the worlds broadcast was a social experiment as well, which could be true or false. idk. and I don't have any source for this statement about Keel beyond Jim Keith's interview but I thought it was worth bringing up.
overall, i don't think that Keel's "search for the Big Fish" discredits his research or his intentions, and I think it just shows that he was as creative as he was intelligent.