The most appropriate time to include the ending of your story in a book report is, well towards the end. You don't want to end your whole report with just the ending of the story. Tell about the ending of your story, then add your commentary to finish the report. Just keep in mind that when you are writing about the ending of the story in a book report, you don't want to give too much away because the goal is to try and convince your reader to read the book themselves. Also, when you add your commentary, stick to things you observed throughout the book, such as underlying themes, morals, character flaws, etc.
