The Book
Explanation
The Joke
A character reads an entire book on a complex topic and proudly considers themselves educated on the subject. An expert in the field explains that one book is roughly 0.1% of what you'd need to read to actually understand the topic, and that the book the character read probably oversimplified, omitted crucial nuance, and may have been written by someone with an agenda.
The character says they still feel informed. The expert says that's the problem.
The Humor
The comic targets the specific failure mode of reading one popular book on a subject and concluding you understand it. Popular science and popular economics books are necessarily simplifications, and many are advocacy dressed as explanation. Reading one book is better than reading none, but much worse than understanding your own limitations.
Context
This connects to the Dunning-Kruger effect: a little knowledge can make you feel more confident than a lot of knowledge, because the more you know, the more you appreciate what you don't know. The comic also comments on the popular nonfiction genre, which has been criticized for presenting complex topics as simpler and more settled than they are (Malcolm Gladwell being a frequent target of this criticism).