The Joint Special Operations Command (JSOC), which includes the classified units that killed Osama Bin Laden, has another secret mission according to a new book by Marc Ambinder and D. B. Grady — protecting the continuity of civilian government in the event of a coup.

"JSOC is also a key part of the classified contingency plans to preserve the U.S. civilian government in the event of a coup from the military or anyone else."

JSOC, which includes SEAL Team Six, and the Combat Applications Group, known colloquially as Delta Force, as well as other units of greater size and even more secrecy, is "the President's secret army," the authors contend.

"When the president travels overseas, a JSOC team usually shadows him. Its members are trained to take charge should the mammoth security structure of the U.S. Secret Service break down."

The Secret Service did not immediately return a request for comment on the book's revelations.

The book, "The Command: Deep Inside the President’s Secret Army," provides the clearest view into the secret world that is JSOC, and it's growing use as a tool in American foreign policy and homeland security.

President Barack Obama has increasingly relied on JSOC units to carry out counterterrorism operations in Pakistan, Somalia, and around the globe — usually with little or no congressional oversight.

"The Command: Deep Inside the President’s Secret Army" is available on Kindle and iBooks.