RADIANT ANGEL, by Nelson DeMille

Book seven in series. Further adventures of John Corey, feisty former New York City detective and now part of a task force monitoring the activities of the Russian espionage agents working in New York.
Corey, whose short fuse is legendary as well as the despair of his FBI wife, tracks members of the Russian delegation to the U. N., among them one deadly agent of the KGB. Russian colonel Vasily Petrov of the Russian Foreign Intelligence Service goes to a weekend party at the eastern end of Long Island, playground of the rich and devious. Corey, of course, violates protocol and masquerades as a caterer to follow the Russian into a wealthy oligarch's stronghold.
When Petrov and a dozen beautiful escorts walk down tthe beach and disappear from view at the height of the party. alarms go off in Corey's head but nobody else is disturbed.  Rejecting the conservative approach of asking his superiors for direction, Corey and a reluctant counterpart with mysterious connections follow the fast-vanishing trail.
If you enjoy edge-of-the-seat, high body count tales, DeMille is the author for you, and John Corey is the character, in every sense of the word. Confrontational, confident, smart-ass Corey will keep you entertained as well as terrified.
It's a 5. The series is best read in order as there's quite a bit of carry-over.