My opinion of this book is just that - total opinion. But that's fair, considering that the book itself is opinion. James offers very little documentation. The author asserts:
*There is an increase in unhappiness in Western Countries.
*Denmark is less unequal, has better socialization, and people are happier.
*The cause is rampant capitalism. We are even taught in schools (in so many word) to become good little consumers and producers.
*blah blah blah blah
But James has a point - many of the frustrations of living today we didn't have 50 years ago (or ever). That doesn't mean you can create an epidemiologic study that will reliably establish causes and effects. That would be no easier than forecasting the weather beyond a few days - there are too many variables.
Happiness is a very individual, elusive, and hard-to-pin-down combination of emotions. There is much evidence that genetics governs the biochemistry that controls a person's general outlook - perhaps realistically thought of as one's "happiness thermostat." Some studies confirm that after good or bad life-changing events, people tend to eventually (sooner rather than later) return to their inherent steady state level of happiness. Nurture, on the other hand, is judged much more influential about learned behaviors such as personal habits - and good ones are bound to benefit one's happiness.
Our general level of happiness on a day to day basis is more likely to suffer from nitpicky, seemingly insignificant irritants rather than how generally well off we are otherwise. Old saying such as "Don't sweat the small stuff," seems to hold up well here, as does, "Have the serenity to accept the things I can't change, the courage to change those I can, and the wisdom to know the difference." It makes sense to try to realistically identify and change recurrent irritants - also to re-evaluate the things that one REALLY likes, and make the appropriate adjustments in lifestyle.
With that in mind, James scores a point. The minor irritants may be more numerous today - more junk mail, computer viruses, too-complicated electronic devices, recurrent new charges on monthly bills, inability to find a human to talk to on the telephone, too many soccer games, etc. Depending on how you handle the potential sensory overload, your general happiness can suffer, but it's up to you, as always.
Overall, I'd say the book is worth reading if you don't mind sermons. If you follow his advice - don't compare yourself to others, don't get in debt, be less materialistic, make an effort to be emotionally mature - it certainly can't hurt. But rather than blaming capitalism (not that rampant capitalism doesn't have its faults), the finger could point instead to huge masses of people having to share (relatively) less space during the age of the internet and high technology. And you don't have to participate in all of it to be happy. All in all, I don't think there's ever been a better time to be alive.