Apple is currently giving out freebies to Books users in the US. The next time you launch the app, you'll see a notification inviting you to "explore free books, read-alongs for kids, cozy mysteries, and audiobooks for the whole family." Tapping on the message will lead you to a featured collection where you can select one book or audiobook from a variety of genres. You can also pick a novel from the company's "First in a Series, Free" section.
The timing of the promotion suggests Apple is trying to help out people who are stuck inside during the coronavirus pandemic. This appears to be a limited-time offer, so if you're interested, take advantage as soon as you can.
Once you've finished reading your freebie from Apple Books, you can turn to an app like Libby to get more reading material. Libby allows you to borrow e-books and audiobooks from your local library. All you need is a library card, which, in most instances, you can get for free.