I haven't read the book and the Pentagon is being tight lipped about what may be considered classified in the book from everything I've heard/read. But I'm guessing it's more about the training a SEAL undergoes, the protocols they have to follow in combat, etc.. Things that aren't made public knowledge simply because you don't want the enemy to know how you are trained to react in combat situations.
A Rear Admiral had this to say about the matter:
Meanwhile, in an internal message to his command entitled "The Cost of Disclosure," Rear Admiral Sean Pybus, who heads Naval Special Warfare Command (NSW), criticized SEALs whom he said had violated the command's ethos.
"We do NOT advertise the nature of our work, NOR do we seek recognition for our actions," Pybus wrote to his command in the letter, a copy of which was obtained by ABC News.
"I am disappointed, embarrassed and concerned", writes Prybus. "Today, we find former SEALs headlining positions in a Presidential campaign; hawking details about a mission against Enemy Number 1; and generally selling other aspects of NSW training and operations."
Aside from hurting NSW's reputation and security, Pybus said "the security of our Force and Families is also put at risk by the release of sensitive information" and said enemies can gather information and NSW details that "expose us to unnecessary danger."
If this had been an expose of injustice or corruption within the military, it would be one thing. But this was just a blatant attempt to cash in by this soldier.