American manufacturing dominated most of the 20th century as its methods of mass production took over the world. Nowhere was this dominance more clearly on display than in the auto sector, where guys like Henry Ford and Alfred Sloan were driving down the cost of owning a car while driving up what a car could do. But the tide started to turn when some Japanese manufacturers came up with a better way. The Machine That Changed The World is about that new way.
The common perception of the time was that Japanese manufacturers benefited from lower wages. To an extent this was true, but it did not tell the whole story. Can cheaper labour be the cause of fewer defects and higher quality? Maybe, if this labour was employed to test and stamp out any defect before it could reach customers. But this wasn't what was happening. Companies like Toyota were able to make cars with fewer people than the American manufacturers, thanks to the concept of lean manufacturing.
Womack and his team went to a number of plants both here and overseas in order to study what was going on, and this book describes that work. He discusses the concept of lean, not just as it pertains to manufacturing but also to marketing, distribution and other functions, and how it came to be via the evolution of car-making.
If you're interested only in the concepts of lean manufacturing, I would instead recommend the book The Goal, as it does so more eloquently. If you want the history of auto manufacturing as well, however, then The Machine That Changed The World is for you.