The most covered topic is eugenics and population control (still having a large impact today, and being funded by US tax dollars) and also how that relates to the Nazi Holocaust. It also covers attacks on life-giving technologies like nuclear power, DDT and genetically engineered crops. And it covers the attack on economic development and "fire" (combustion) from global warming rhetoric and the anti-human plans for dealing with global warming by destroying modern industrial progress.
The book is strong and shocking. The brutality of some of these things, and callous destruction of human life, can be hard to take. But it's stuff everyone should be informed about. This book is pretty easy to read. I think it's a good start on the topic. It's badly researched, but the better books are less appealing in other ways. I'd suggesting reading Merchants without trusting any specifics and then reading other books to learn more detail. Merchants covers a lot of ground quickly and enjoyably and can serve as a good introduction to make you aware of its topics. I think most of what Merchants says is approximately, mostly true, besides the Malthus stuff.
It's a philosophical book. The overall theme is the ideas of antihumanism, which it traces back to Malthus and Darwin ("social darwinism" is a mistake and not implied by the theory of evolution, but it was Charles Darwin's open intention), and then to the Nazis and the modern population control and green movements.
Population control is the new name for eugenics. It claims that we have limited resources and too many people. Does that remind you of today's green sustainability agenda? It's evil but it's popular. Actually people use their minds to create resources, and there's plenty of raw material to turn into resources. We're better off with more people, not fewer; human life is a good thing. But because of this nonsense, US money (and other) is going to sterilize vast numbers of people, often without consent and in unsanitary ways. It's especially affecting poor people and frequently targeting ethnic minorities around the globe. The extreme brutality of China's one child policy is only a small fraction of it.
Don't understand what I'm talking about? Don't worry. Read the book and it will explain it all in gory detail. It's pretty short and easy to read. The stakes are huge. Anyone who cares about humanitarian issues must inform themselves about these problems. Start here or elsewhere, but do start!
Read a sample about population control.
Buy the book.
EDIT: I have edited this post after finding multiple scholarship errors (e.g. misquotes, incorrect historical facts) in Merchants of Despair. For more information on the scholarship mistakes, see my blog post about it.