I'm writing a personal essay about a novel. Every time I use the title of my novel in the my essay my teacher wants me to underline it. For one part of the essay I'm supposed to talk about if the author has published any other novels. I was just wondering if I should underline the titles to the others novels I talk about that have been published by the author?

Yes, the title of any book should be underlined, all the time. Proper names of books, films, etc., should always be underlined (or italicized in print). The name of an article from a magazine or a book should be in quotation marks, but the title of the book, magazine, newspaper, etc., must be underlined.