1929 stock market crash and unemployment rate
One caveat to the author you refer to, Thomas Sowell is a staunchly conservative economist, so most arguments you hear from him will stick to the Econ 101 wisdom that government intervention is usually bad, tariffs are always bad (to be fair, it is hard to use them to correct externalities for global "public goods"), etc.

Although tariffs probably were not necessarily the right choice for a tightening economy, the general story is that our central bank in America back then was a lot less independent from politics and less knowledgeable of how to deal with liquidity crises. It ended up tightening monetary policy instead of loosening it to ease the economic contraction, exacerbating employment problems for a long time. Keep in mind as well that this period of unemployment over 10% lasted for over a decade or so; it's not quite feasible to tell what all the factors were that caused the high unemployment, especially without official data on unemployment for a lot of that period.