It's gotta be said, this book is a product of its time (nearly 30 years ago) and context (British, paranoid). Does it have useful basics that are still applicable even after things like 9/11 and smartphones? Yes. But you can't ignore how technology leaps and the changing context of the world in the next decade has to have changed bodyguarding, among everything else.
That said, it is a fantastically good historical primer. I learned a lot about a bunch of people and events. It looks to have good insider information, and the stories are engaging, never dry. A good read over all.