The Last Kingdom by Steve Berry

From celebrated New York Times bestselling author, Steve Berry, comes the latest Cotton Malone adventure, in which the discovery of a lost historical document challenges the global might of the United States. King Ludwig II of Bavaria was an enigmatic figure who was deposed in 1886, mysteriously drowning three days later. Eccentric to the point of madness, in the years before he died Ludwig engaged in a worldwide search for a new kingdom. One separate from Bavaria, a place he could retreat to and rule as he wished. But a question remains: Did he succeed?
Enter Cotton Malone. Luke Daniels, Malone's Magellan Billet protégé, has managed to infiltrate a renegade group on the trail of a 19th century deed that proves Ludwig's long-rumored search bore fruit. A document that could not only secure Bavaria's independence, but also change the balance of power in the modern world. The United States, China, and Germany all want it.
In a race across Bavaria for clues hidden inside Ludwig's three fairytale castles, Cotton and Luke battle an ever-growing list of deadly adversaries all intent on finding the last kingdom.