Hiya, Rue!

Great question. What you’d do is use the USPTO ID Master List. Basically, you just search for the type of goods or services you’re offering and go from there. For example, if you want to trademark a series of books, you’d search for “book series” to display a bunch of relevant categories. Just keep in mind that some are for print books and others are for electronic books, so it’s always a wise move to secure both.