In the John Clark/Jack Ryan timeline, I do believe the next book you should get is Patriot Games. After that, The Hunt For Red October. But to be sure, go to Tom Clancy's website. He's got them listed in order of which they were written and also listed in the John Clark (Kelly)/Jack Ryan timeline, as well.
I really like Vince flynn's Mitch Rapp series. He's more like a cross between a Jack Ryan and a John clark. or a Jack Ryan with some serious skill sets and amazing tradecraft. Language is probably rougher than the Jack Ryan stuff but I've been thorugh about 4 of them so far and like them as much or more than clancy.