After you run a search for your topic in the Discover Search, to find a particular type of resource (book, ebook, magazine article, academic journal article, etc.) use the "Source Type" limiter option on the left side of the screen. It will look like the picture below:

If you don’t see the source type you’re looking for, click “Show More” and a window will open up with all Source Type choices, as shown in the picture below.

For more help, ask a librarian.