Secrets of the Moon

Agatha Witchley used to be a spy in the Cold War, but now she's locked up in the UK's premier maximum-security mental institution. She believes that the ghosts of the celebrity dead visit her padded cell and whisper the world's secrets in her ears. Which is a big problem for the British government, because she's the only one who can help them when an American billionaire is murdered in London in one of the strangest killings yet. 

The Home Secretary needs the case locked down and solved before the entrepreneur's death becomes public knowledge and economic chaos ensures. 

The woman he has in mind for the job might be paranoid, she might be lethal, she might half-insane and drawing a pension, but it's amazing how you can forgive that in a genius when it's a genius's help you need.

Yes, the security forces need Agatha Witchley again. It's just the ghosts of Churchill, Elvis and Groucho Marx they could do without.