Short answer: GEO means optimizing content so generative models like ChatGPT, Perplexity, or Gemini use it as a source when building their answers — which translates into a reader asking an AI about your topic, and the AI mentioning your book.
More and more people use AI chats as their starting point for recommendations (books, tools, services), instead of typing directly into Google. If your content isn't built with this in mind, it simply doesn't exist for that discovery channel.
SEO wins the click; AEO wins the direct answer inside a search engine; GEO wins the mention inside a conversation with a generative AI — which can happen without the person ever visiting your site.