"“No Easy Day"/"50 Shades of Grey" (File)

“No Easy Day,” the controversial book by a former U.S. Navy SEAL about the commando raid that killed Osama bin Laden, has stormed to the top of U.S. book sales on online retailer Amazon.com.

The unauthorized book, which has caused the U.S. Government to threaten legal action on the grounds of non-disclosure of government secrets, is already out-selling the hugely popular Fifty Shades erotica book series.

The hardcover edition of “No Easy Day” officially becomes available in the United States on Tuesday, September 4. Amazon.com’s best-seller list includes pre-orders.

“No Easy Day: The Firsthand Account of the Mission That Killed Osama Bin Laden,” is written by a Navy SEAL operative under the pseudonym Mark Owen.

His real name, Matt Bissonnette, was revealed shortly after news broke that the first-hand account of the daring operation on the al Qaeda leader’s compound in Pakistan last year was to be published. Reuters obtained an advance copy of the book from the publisher, Dutton, an imprint of Penguin Group USA.

The Pentagon has threatened legal action against Bissonnette for alleged violation of non-disclosure agreements because the manuscript was not submitted for a pre-publication security review. Bissonnette’s attorney responded that the subject matter of his book was not covered by non-disclosure agreements he has signed.

Bissonnette wrote that he decided to do the book because details of the raid that were being leaked by others in government were wrong.

The controversy around “No Easy Day” will be further amplified by a follow-up e-book called “No Easy Op” co-authored by unidentified former Special Operations members from a group called Special Operations Forces Situation Report, or SOFREP. The book claims to provide analysis of the politics behind “No Easy Day” and to provide new insider information that has not previously been provided to the media.