Textbooks are didactic. Non-fiction is autodidactic. This is by design: "what math do I need in order to understand physics?" is an externally-guided search. It also holds true for liberal arts: "What songs should I listen to in order to understand the influence of music on American culture?" is not a question you can solve yourself without a whole bunch of research and exploration. However, "what are the cool parts of physics?" becomes an autodidactic search because "cool" is a controversial qualifier. Likewise, "I want to know more about Bruce Springsteen" is not a quest that should be informed by committee. This is an autodidactic discussion. "I'm interested in exploring my historical blind spots" is not something that should be answered with textbooks. It's like suggesting someone learn more about the events that led to September 11 by reading this thing.