You really can't go wrong! All Parker is good Parker

SHARPS is the most like The Folding Knife, and if you want to continue in a very similar vein (but with more swordplay) that's a good next book... it is a mystery about treachery and politics still, but also has some convoluted and disturbing characters. And swords.
Scavenger is (possibly) my favourite of the trilogies - it can be a bit of a slog, be warned, but it is incredibly tricky and really, really wonderful.
Cons: it is extremely elaborate, and has a lot of odd twists, and it plays with concepts of memory, so it can be kind of a headache. Also, literally, it ends with the very, very, very last paragraph of the very, very, very last book. So you'll read 1000 pages, and everything will resolve in a single line. Seriously. So it is best read as a very long standalone, if that makes sense. You won't want to take breaks between books.
Pros: really cool setting - a sort of minimalist fantasy world. One of the most badass reinterpretations of swordplay ever - seriously, it turns everything on its head when it comes to combat, and I'd take one of Parker's sword-monks over anyone else in fantasy. A fantastic atmosphere, and, as noted above - loads of twists.