For people who don't want to read all of it, the author has researched and found that people have thought that america was declining for pretty much the entire history of our country, and that happens to be a good thing because it forces us to act on real or perceived threats.
After interviewing many historians, economists, and other experts he concludes that american society is as healthy as it's ever been, but the real problem lies in our government. The government is too old, and has become so inflexible that it is unable to adapt quickly enough to meet any threats, or focus on what needs to be done. Special interests have their claws dug so deeply into every level of the government that doing nothing is the best and easiest option for politicians. The next problem is the senate, which if you remember from your american history class, was a compromise in creating the union for the smaller states to have as much say as the larger states. This has hamstrung our country at this point, if you remember from the current hoopla about health care, the "gang of 6", or 6 senators who ultimately had the most control, only represent like 3% of the population of the country, and that is wrong. Just as 50% of the country is worth only 20 votes in the senate is also wrong.