Fidelity by Jan Fedarcyk Simon & Schuster, $25.00, 306 pages, ISBN 978-1-4767-3386-9 With some seven and-a-half million residents spread out over a couple thousand city blocks, the Manhattan Office of the FBI is one of the largest, busiest and most important of all postings within the agency. The activity there is ceaseless, and the agents are all at the top of their game. They’re the best of the best