Clark's book is an ambitious look at world economic history. The first half of the book is an excellent discussion of the Malthusian trap, in which increases in standard of living only lead to increases in population, which then (over generations) lead to declines in standard of living. The only stable point of this dynamics is at subsistence-level income. I need to think more about it, but I suspect Clark overstates the case for how well the Malthusian model applied in early human history. My impression is that there were wide disparities in levels of development that can't be easily explained in that context.
The second half of the book concerns the industrial revolution, and advances his (controversial) thesis that one of the main causes for this qualitative shift in the rate of human advancement was genetic. By analyzing historical demographic data he argues that by 1800 almost all residents of England were descended from previous generations of wealthy strivers -- reproduction rates correlated highly with family wealth in the previous Malthusian era, and the wealthy literally replaced the poor over time (less favored offspring of the rich often become the poor of future generations). Therefore, traits which are positive for commerce, long term planning, wealth accumulation, market organization, etc. had become much more widespread thanks to natural selection. I find this effect plausible -- it is consistent with recent genetic data on accelerated human evolution -- but am not as convinced that it dominates over cultural factors (at least, the two would work hand in hand). His case that it was a priori likely for England to be the first to have an industrial revolution doesn't seem particularly convincing (see Kenneth Pomeranz's Great Divergence for another set of arguments based on geography and natural resources).
Clark makes the interesting connection between modern man's descent from the strivers of previous generations and the hedonic treadmill: our happiness seems to correlate more with our position relative to perceived peers than with absolute levels of wealth.
I like the following quote from the final chapter of the book. I always found it very amusing that modern economic models can't do much better than to treat technological change as an exogenous, stochastic variable. (Yes, I know about Romer and growth theory, but would lump that in the "can't do much better" category.)
God clearly created the laws of the economic world in order to have a little fun at economists' expense. In other areas of inquiry, such as the physical sciences, there has been a steady accumulation of knowledge over the past four hundred years. Earlier theories proved inadequate. But those that replaced them encompassed the earlier theories and gave practitioners greater ability to predict outcomes across a wider range of conditions. In economics, however, we see instead that our ability to describe and predict the economic world reached a peak around 1800. In the years since the Industrial Revolution there has been a progressive and continuing disengagement of economic models from any ability to predict differences of income and wealth across time and across countries and regions.