As more and more of us spend money on fitness, Sweat Equity: Inside the New Economy of Mind and Body explores the investors and entrepreneurs making money from this booming market. While she would have welcomed more reflection on how this lucrative world is tied in with the rise of the ‘quantified self’, Jason Kelly shines a fascinating light on the US fitness industry in this well-researched, informative and eye-opening book, writes Ella Hafermalz