Can I write a book from popular YouTube videos or podcasts?

Yes, as research. View counts are a good sign of what readers want, and Listen turns a long video into a summary, key points and chapters you can read in minutes. File those videos as research sources (the default for links): BookyAI then plans an original book on the subject and writes it in your own words, crediting facts to their source. Do not republish someone else's content as your own; the app is set up to avoid that, and you remain responsible for what you publish.