This guided notes handout can be used to a one day lesson on author's purpose. It can be edited to be used with or without an accomanying video, with different books, or however you need it to fit your needs. I used this for my observation last year as a middle school librarian and the students definitely enjoyed and learned from the lesson.
Attached is the link to the video I used: https://www.youtube.com/watch?v=03ap0LdT4zk
If you have any questions, feel free to email me!