1. Tap the Share ⬆ button at the bottom of the screen (on iPad, at the top).
2. Scroll down and tap Add to Home Screen ⊞.
3. Tap Add. The book's cover appears on your home screen.

You opened this inside another app. To add it to your phone, first open it in your browser:

1. Tap the ⋯ or ⋮ menu in the corner.
2. Choose Open in browser (or Open in Safari / Chrome).