With out any doubt, this is the most terrifying book I have ever read. It talks about modern industrial societies' reliance on petroleum, and the ramifications for those societies now that oil availability has peaked and will forever become more scarce.
The short answer as presented by the author in a series of matter-of-fact analysis of the data and the various theories that have been built around that data is that we are in trouble. Not just 'this will harm the economy' trouble, but 'I will have to learn how to live without electricity, modern medicine, transportation and government' trouble trouble like 'I will need to learn how to grow my own food and hunt'.
The long answer, as pointed out in convincing and clear fashion, is that now that are in the era of declining oil production, our entire economic system even the basis of our currencies is in trouble. The basic reason for this is that the underlying concept underpinning our economic system is 'perpetual growth'. This system has worked, the author argues, because we've had massive amounts of essentially free energy in the form of petroleum. Now that that free energy is no longer free, perpetual growth is impossible, and economic contraction to a new, stable system based on local economies underpinned by renewable energy sources.
The usual problem with such doomsday advocacy is that the arguments tend to fall apart under anything more than casual inspection. But Heinberg is merely the reporter in this case, and the body of work he's pointing to is that done by people such as petroleum geologists. Scientists. And not just ivory tower scientists, but people with hands-on exposure to the matter they're studying. And all of them agree on this subject - all of them.
I strongly recommend this book to anyone who can read English and plans on being alive in the next few years.
(P.S. I don't know why every book title has to be so very long these days. As ridiculously long as this one is, it seems to be par for the course.)