Each year there are new fads in writing. Sometime last year I noticed an increase of ellipses, em dashes, and italics. Personally, I’m with the old school of only using them sparingly. Sparingly means very few through out a book.
Most ellipses I see used can be avoided. Ellipses are for a dangling sentence, or when someone interrupts the character speaking. Maybe a trailing thought. These are not to be used in most sentences, but a few strategically spaced through the book.
Em dashes are the same. Use sparingly. The em dash is explained in The Chicago Manual of Style in sections 6.82 through 6.89. There are several different uses for them, but again they should be used sparingly.
Italics have been used to show an exclamation, but should only be used a few times through the book. Most readers know when the exclamation is used in a sentence and don’t have to be shown. In a movie script, I would say use it so the actor knows when to inflate his/her voice.
Italics are also used for deep thoughts and deep prayers. Every thought is not italicized. Italics slow a reader down and as a writer, you don’t want your reader to stop and think about what they just read, and possible read it over again. If that is done very many times, the reader will put the book down.
When you see the above in a book, you may think it is something you want to use, and I encourage you to do so, but do sparingly. Your work will look much neater and be easier to read. Just because someone else is using it doesn’t mean you have to. Write well without fads.