The Lost Order by Steve Berry
Steve Berry is back and has written a fascinating book in the Cotton Malone series that fictionalizes (but nowhere near as much as you think) about the Knights of the Golden Circle, the largest and most dangerous clandestine organization in American history.

In The Lost Order, the battle lines are drawn:

“The Knights amassed billions in stolen gold and silver, all buried in hidden caches across the United States. Since 1865 treasure hunters have searched, but little of that immense wealth has ever been found. Now, one hundred and sixty years later, two factions of what remains of the Knights want that lost treasure—one to spend it for their own ends, the other to preserve it. Thrust into this battle is former Justice Department agent Cotton Malone, whose connection to the knights is far deeper than he ever imagined.”

BTW … one faction also intends to upend the Constitution with a lawful twist that will have you wondering whether that might really be possible.
History lies at the heart of every Steve Berry novel. It’s his passion, one he shares with his wife, Elizabeth, which led them to create History Matters, a foundation dedicated to historic preservation. Since 2009 Steve and Elizabeth have crossed the country to save endangered historic treasures, raising money via lectures, receptions, galas, luncheons, dinners and their popular writers’ workshops.
Here is the trailer and TV commercial for The Lost Order.
As always for a character-driven series, I recommend starting with Book #1, The Templar Legacy.