Well, I don't know about the reality of it, but movies pretty much do a good job of screwing Clancy's novels up pretty good. Hunt for Red October was a fantastic book and after reading it, I can barely even watch the movie (Sean Connery as a Russian with a Scottish accent didn't help). Patriot Games and The Sum of all Fears also are equally screwed up. But as was stated in a recent posting, his books are going downhill too.
Why do all these authors think they have to have the same characters in every book? I mean, Jack Ryan has got to be about 90 now.

It doesn't have to always be a sequel. In fact it might have helped some to NOT be a sequel...