Did you know that Valerie Plame worked for the CIA? Yeah, I didn't either. Who knew? Anyhow, she wanted to write a book about it. She couldn't, because it was classified information. See Wilson v. CIA, No 07-4244 (2d Cir. Nov. 12, 2009) (here) (no First Amendment right to talk about stuff that everyone knows about, because said stuff is classified information).