One other book that offers a lot of insight is named 'Balance - from Ancient Rome to Modern America'.
When I see books that expound at length on the fate of the US, the first thing I use to figure out whether it's a hack job is to look at how quickly the content turns into invective. I've found books that start on page one by blaming some group of evil people in the midst of a conspiracy, and it doesn't stop - ever. This is not one of those books.
One point made in this book is that Rome never 'fell', it simply dissolved. It wasn't replaced by some other empire, as it fell apart it simply deteriorated into isolated chunks. However, there is a purported natural disaster that occurred around 535 AD, this is presumably a 'dark age' as the result of a volcanic eruption, probably in Sumatra. The few years after this event were known for starvation, cannibalism, mass migration, and other evidence of serious ecological disturbance.
One point the book makes is that governments tend to get in debt, which simply gets worse over time. True for Rome, France at the time of the Revolution, most 'Western' countries today, and others. Generally the trends (as the book points out) are that government benefits become more generous, groups split up into more and more assertive special interest groups intent on protecting their privileges, and the sovereign state turns 'inward' - building walls instead of roads, excluding foreigners and expelling 'undesirables' instead of absorbing fresh blood from afar.
As the US debt approaches the capacity of taxpayers to keep up, the credibility of the central government declines. In short, if you know someone that has cash, you believe them when they say they're going to do something. If you know someone is broke but can borrow money, you suspect they aren't going to attempt anything as ambitious as someone with money. If you know someone that is not only broke but in such hock that no one will loan them any more, you know they're basically trapped, and not going anywhere. This is the likely fate of most modern democracies. At some point people will simply quit believing government has the capacity to act. One would expect this to lead to moral decline, but other institutions tend to spring up to take it's place, and often these are religious.