We use 'on' for non-fiction, schoolbooks and academic texts. If you say "a book on electronics", for example, this is the subject of the book. We also use on for the subject of lectures, documentaries and so on. We can use 'about' for any type of book. While you can say either "a book on electronics" or "a book about electronics", we use only 'about' for non-fiction: for example "a book about a little girl who flew to the moon". We wouldn't use 'on' for the subject of a storybook.

Both denotations express having (something) as a topic.