No Higher Law — Philip Friedman

Prosecutor Ben Kaplan, New York chief of Major Crimes in the US Attorney's office, has always known exactly where his loyalties lie. But when his investigation of commercial fraud by a group of orthodox Jews uncovers a brilliantly counterfeited hundred-dollar bill, he enters a world where all his certainties are challenged.
Struggling to deal with his wife's death three years earlier, Ben has finally found comfort and meaning with the charismatic rabbi Joshue Brauner and his enchanting follower Judith Zilka. But as the counterfeiting case turns into something far more sinister and dangerous, Ben begins to fear that his prime suspects may themselves be disciples of Brauner's.
Suddenly Ben is faced wth the most testing decisions of his life: if he cannot tell truth from lies, good from evil, he risks the very lives of the ones he loves the most!