This is book number 11 in the Parker Novels series.
The good guys in a new African nation were missing a treasury full of diamonds. The nation's corrupt leader had stashed the rocks somewhere in New York City. Now the good guys needed a specialist to get their diamonds back. So they came to the best in the business: Parker. Only the three mysterious tough guys came to Parker, too. They figured three hands filled with .38's could convince him to pass up this international gem game. But leaning on Parker was like pressuring a box of TNT with a short fuse.