Their path to immortality is via 3 stages (they call them bridges). The first is a healthly lifestyle, which will keep us alive for another couple of decades. By then we would hoped that the next bridge, biotechnology, will have been reached, to prolong our lives even more. Thus we should reach the third bridge, nanotechnology, which offers the promise of making us immortal.
The majority of the book is concerned with the first bridge. The authors look at what we eat, showing what we should eat more of an what we should avoid - sugar is particularly bad it seems. They also discuss what supplements we should take, and the benefits of exercise. There are chapters on illnesses such as cancer and heart disease, and what we can do to avoid them.
I think the book will be of interest, even for those not following the recommendations, for the detail that it goes into (which might be too much detail for some). For instance it explains the difference between omega-3 and omega-6 fats, and why the first is so much better for you. I'm not convinced I would want to follow their regime myself - too many tablets to take for one thing. (Although, as they point out, this will only be necessary for a couple of decades, which will be an insignificant part of our potential lives). But it's always useful to know what options might be open to me.