This is a pretty stunning book. Not perfect, by any means, but still really important.
The basic premise is that we are about to (or already have--the book predicts 2010) hit peak oil, and that really, none of the renewable sources of energy will ever be close to being able to provide enough energy to make up for the loss of cheap oil. The consequences of this will be drastic (and, since we've not prepared for it in any real way) catastrophic changes in our economy and society.
However, just because we're going to run out of cheap oil does not mean we're going to fall instantly into a Mad Max world of muscle cars and mohawks.
Greer's thesis is that the rising price of oil will cause repeated shocks to the economy. He's not talking shocks that cause a 10% drop in the Dow and a 10% rise in bank bailouts. He's talking about the collapse of substantial sectors of the economy that depend on cheap energy--like mail order and importing tchoschkes from China. Each shock will be followed by a period of stabilization, as things adjust to the new energy reality. Eventually, however, the continuing depletion of energy reserves will cause another shock, as the cycle repeats. Eventually, we end up in much-less-populated world, with--if we're lucky--a sort of enlightened 18th century society based primarily on manual labor.
The key here, though, is that extreme visions of the future--either a happily chugging along utopia of sunshine and electric cars and online medical care for everyone, or a dark, brutal world with only the strongest surviving off the roasted legs of their second cousins--are equally unlikely. We're running out of energy, it's true, which makes the sunshine and flowers future unlikely, but history seems to show that when societies fail, they don't collapse all at once, but rather in fits and starts.
Which means, instead of an eternal rise or a sudden fall, the titular long descent. So what do we do about it? Well, on a societal level, Greer suggests we've missed our best chances by abandoning the energy-conscious advances we made in the 70s for SUVs and server farms. We can start now trying to conserve energy, and develop alternate sources, but our best opportunities are behind us. On an individual level, however, Greer suggests developing skills now that will be useful in a localized, post-oil economy. Your Python coding experience, for example, won't be so useful. Your woodcarving, or ceramics, or appliance repair experience, on the other hand, may be.
Greer has taken a fair amount of criticism by folks claiming he's not taking into account climate change, and that he's looking at things all to rosily. It's certainly the case that climate change is barely mentioned, but the near- and medium-term effects of climate change are likely to be swamped by the effects of an economy bereft of cheap power. It's possible that the effects of climate change could be the extra push which means the difference between a Long Descent and a Sleigh Ride to Hell, but it's certainly not self-defeating to neglect to devote a chapter to it.
A more serious problem, I think, is the religious axes Greer has to grind. While he's not really proselytizing (Greer identifies as a druid, and admits that his faith is unlikely to be the spiritual path that the masses follow through the long tough times ahead), the author is insistent that only religion (and religion in an organized, churchy sense) will see people through the Long Descent. Our faith in Progress, he writes, will be shown to be a false faith, and what's left is a faith in God. I think he's missing an important distinction here. I think that he's mistaking faith in Progress in the material sense for a sense of Progress in the sense of treating each other better. Only the most rigid of economic determinists would find a notion of technological progress at the heart of, say, the civil rights movement. Was it religious? Yes, in part. But it was also driven by a secular notion of equality, rooted neither in God or in Progress in the technological sense. In a way, Greer repeats the errors of the dogmatists he decries, by straining to fit a multivalent notion into a small, defined box.
Overall, though, this book is immensely thought-provoking, and if not actually of immediate practical use, certainly helps me conceptualize how to face the coming calamities.