The title of the book poses a great question, but unfortunately the book itself makes very little attempt to actually answer it. There are some interesting anecdotes on a number of different topics, with no real coherent theme to bring them together.
I would imagine the book will appeal to those with little knowledge of the subject matter. Peston's writing style certainly makes the subject matter approachable. However attention to detail is poor. How can anyone have properly proofed a book that discusses Norman Lamont's 2003! budget, for example in Chapter 7. Some of the "factual" matter is simply embarrassingly wrong. For example when discussing Philip Green's possible takeover of M&S in Chapter 4, Peston cites a sharp increase in the amount of M&S shares borrowed as an indication that "astute investors had a whiff that a takeover bid was coming". You borrow stock so that you can SELL it and profit from the price FALLING. If you think there is going to be a bid then you would be buying, not selling and you have no need to borrow stock. If the author is unable to differentiate between buying and selling it does make you question which other "facts" were actually checked.
On completing the book I was left with the abiding thought that in these interesting economic times, those who really understand what's going on make money from calling the way the markets will move, and those who don't understand make money by selling books.